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Thread: 9mm vs .45ACP effectiveness explained

  1. #1
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    In a local gun store I recently encountered the most ridiculous 9mm vs. .45ACP story I think I’ve ever heard. The salesperson was talking to someone who I guess had never owned a handgun, and was considering doing so. Perhaps they couldn’t decide what caliber would be best, but regardless at some point in the conversation they asked the salesperson about it. I don’t know if the customer was the one that specifically brought up the 9mm vs. .45ACP thing, or if the salesperson simply decided to only talk about those two calibers. The salesperson’s statement was essentially as follows:


    The 9mm has about 500 “lbs” more energy than the .45ACP, because it is a supersonic round. The .45ACP has less energy because it is a subsonic round. The .45ACP has around 1000 “lbs” of energy (!) where the 9mm, having 500 "lbs" more, has 1500 “lbs” (!!!). But, the .45ACP still delivers more energy to the target because, since it’s wider and subsonic, it ALWAYS stops inside the target delivering all of its energy. The 9mm on the other hand, being thin and supersonic, ALWAYS over-penetrates the target, delivering far less energy to the target than the .45ACP will. In conclusion: The .45ACP has less energy, and delivers more of it. The 9mm overpenetrates through everything, all the time.


    I couldn’t believe what I was hearing. Of course we know that in either caliber you will find subsonic and supersonic rounds. In either caliber you will find rounds with under and over-penetration issues. Top of the line .45ACP ammo will in fact usually have slightly more muzzle energy than top of the line 9mm ammo. Both are usually in the 300-500ft-lb range, although some of the highest energy .45ACP rounds may get up near 600ft-lb. Many times even within the same company, muzzle energies and velocities are measured out of 4” barrels for 9mm and 5” barrels for .45ACP rounds though, making the slight differences likely even smaller IMO.

    I also encountered another salesperson that swore up and down that this 18.5” H&R 12ga I was looking at could not fire slugs. I don’t know why, but this salesperson kept saying they would damage it, perhaps due to the choke he thought it had? I knew this wasn’t the case, and I ended up purchasing the shotgun eventually. Well, right there in the owner’s manual it said that it could fire slugs. I’ve put many, many foster slugs through it. They shoot wonderfully out of it, and usually it stays loaded with Winchester Ranger Low Recoil 1oz Slugs.

    Now, this place claims to have some of the most knowledgeable people in the industry working there. I wouldn't expect a whole lot from people at a pawn shop or something, but this is a gun store and they do make the claim that they're extremely knowledgeable. I do like the place as the people are very nice, and I shop there regularly, but at the same time I can’t stand hearing misinformation like that. Granted I don’t think any of these situations would have resulted in a dangerous misunderstanding, but I still don’t see why they'd claim the salespeople know so much if they aren't even testing their knowledge. Perhaps the criteria that's tested isn't related to things like that? I would hope that if they didn't know the answer to a question they wouldn't even attempt to answer it, but that doesn't appear to be the case.

    I haven't been going to gun stores for a really long time, and I've only been to the ones around where I live. Have you guys encountered similar misinformation in other stores? Did they claim to be experts as well?




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    My experience has been that every gun store has one "legal expert" (usually a former cop) and one "technical expert". Sometimes one person covers both roles.

    Considering how widespread this seems to be, I would imagine that these gun stores have found that their "experts" tend to be very effective at selling firearms to certain types of customers (i.e. idiots).

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    "Tales From the Gun Shop"

    These are on every board and you will find input from the following three schools of thought:

    1. Buyer Beware: Anybody that trusts a salesman is a complete fool.
    2. Shop Elsewhere: If you don't like the staff, don't give them your money.
    3. Complain to Management: Their people should be trustworthy, or they should work elsewhere.

    That's it. That's how they ALL end up. Some people hammer on one of them, some people mix and match, and others agree with all three in varying arrangements of importance.

    As for 9mm vs 45... I made a poster for that:



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    :quirkyI haven't seen so much material of thatqualitysince my last trip to a dairy farm.

    Both the 45ACP and the 9mm are subsonic from most pistols, even overloaded. +P+ in 9mm MIGHT be supersonic from a full-size 4.5"-barreled handgun, but for practical purposes a defense handgun is subsonic. A pistol-chambered carbine like the Beretta CX4 can achieve supersonic speeds from a factory9mm, just like a standard .22LRexits a rifle barrel at supersonic speeds, but generally subsonic when used in a pistol.

    Let's talk physics. Force, whichis for practical purposes synonymous with energy,equals mass times acceleration. For the purposes of calculating force, we assume the bullet is totally stopped by its target and therefore force equals mass times velocity. A 9mm factory defenseload might, for example, havea 124gr bullet that exits the muzzle at 1000fps (for simplicity; this is not to much to ask of a standard-pressure 9mm). A .45ACP defense round might have a 230gr projectile exiting at 800fps. That's a projectile weighing 1.85 times as much,traveling at 80% the velocity. Given these numbers, which are realistic but not straight out of the ammo catalog, it's the .45ACP that has more muzzle energy; roughly one and a half times the energy of the 9mm in fact.

    Second, ANY modern defense round between 9mm and 45ACP, including 38 Special and 40S&W, will penetrate between 12 and 18 inches of balistic gelatin. If it did not, it would not be considered sufficient for law enforcement and military use. That's 12 to 18 inches of a material designed to have similar density and consistencyto the human target (on average; densities of course vary widely in the body). This is thereforeroughly translatable; on average, a bullet of any defense caliber will penetrateabout a foot, up to a foot and a half,of human. That depth could be much deeperin the case of ball ammo which penetrates 3 or 4 feet of gelatin, or if the path of the bullet does not encounter bone.That means a human targetof a thickness less than 18 inches stands a good chance of having an exit wound from any defense caliber.

    How many moreobjects are penetrated behind the target depends on a lot of variables, but for simplicity we may say that a bullet exiting a 12"-thick human target retains roughly 1/3 of its muzzle energy. A bullet with 1000 foot-pounds of energy going in may retain 300 foot-pounds when it exists. That's more than enough for a couple panels of sheetrock or to enter a second human target, but an expanding bullet will have increased surface area which spreads the remaining energy of the bullet across a surface possibly more than twice the area of the original bullet's profile and many times that of the initial point of contact with the first target. A ball round would retain more energy having not expanded within its first target (a penetration of 3 or 4 feet of gelatin roughly equates to a ball round retainingtwice as much energyon the far side of a 12" fleshy target as an expander would)and will also retain a similar shape and therefore a similar surface area for distribution of force, placing targets behind the first in more danger than with an expander. That is an advantage of expanders, even though the main reason behind it is that expanders are designed to use more of their energy to make a bigger hole and damage more tissue.

    Regarding shotguns: in general, a 12-gauge pump designed for HD will be capable of firing slugs. In fact, a12Gdesigned forslugs is LESS likely to be able to fire shot than the other way around (many barrels designed for slugs are rifled for greater accuracy and would be damaged by shot). The only general limitation is barrel bore; a true 12-gauge bore will generally accept slugs, while one that is slightly wider or narrower to affect grouping pattern will cause either overpressure failures or, less seriously, reduced power as pressure is reduced by gas escaping around the slug.NEVER use a slug in any shotgunthat has any choke smaller than cylinder.In addition, the pressure of a slug shot is generally higher as the solid mass filling the bore keeps more gas behind the projectile than a clump of shot, so the breech must be rated for slugs and not all shotguns are (though an HD-designed gun could be expected to).

    I have encountered some people working behind firearm counterswhose firearms knowledge is questionable at best.Generally they're found indepartment stores, where the training issimply basic firearm safety and how to fill out the requisite paperwork. These people are to be avoided when you need answers to questions on power, recoil, and versatility.

    The people at my local gun store are, to a person, very knowledgeable about the firearms they sell. You can ask them practically any question and their answer will be close to the truth. There are differences of opinion on some matters; virtually all of them OC a 1911 while in the store, so if you ask for their favorite defense gun they'll point you to those if you have any firearms experience at all. However I've yet to have them steer me away from a pistol for quality reasons (they own the store; why would they sell junk?), nor for defense reasons unless the caliber is patently too small or the gun too large for carry or nightstand.

    They also differ on what's necessary for various game; most recommend a .308 when hunting anything larger than coyote, when IMO a .243 or .270 would be suitable for anything less than elk (and we don't get many of those in Texas). A45ACP handgunis plentyfor coyote, wolf and hog, and in fact I'd prefer a handgun when on hunting on foot for that kind of thing; the handgun's faster than a rifle and can be used in closer quarters, and you're looking for animals that may well fight rather than flee. All of this is really no more than "what would I trust to do the job cleanly and effectivelywithout blowing me off my feet or out of my perch", which is highly subjective.

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    Can you tell me how rifled barrel is damaged by shot? I keep hearing that but I have yet to hear a good explanation. Reminds me how someone was telling me I would ruin my 10/22 shooting shotshells from it. Ran a bunch through it, scrubbed the bore, no problems. The lead just fills in the rifling as I understand, and you just have to clean it good.

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    Well .22 shotshells are not exactly heavy artillery .22 and even 9mm shotshells simply don't have the energy necessary per pellet to cause any damage to the barrel. Now, in some circumstances a rifled barrel can create a wider more random spread, but that's not much of a concern as a shotshell is designed to spread anyway.

    The problem with big shot in a rifled barrel is that the projectiles will bounce around in the barrel as they leave. This is opposed to a slug or ordinary bullet which is a pretty snug fit and, though they will wear the rifling over time, will do so relatively evenly. Shot will not; it will leave small dings in the rifling if a pellet while traveling out the barrel bounces against a rifling channel. Smoothbore doesn't have the problem because, well, it's smooth; it would take an extreme impact to dent it and the angles within the barrel are just too shallowwhen there is no rifling.

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    The speed of sound varies by altitude, varying from 1116fps at sea-level to 1076fps at 10,000ft above sea-level (and decreasing further at higher altitudes).

    Most major pistol cartridgescan and doexceed the speed of sound,measured atthe muzzle of a standard length barrel (~4"). Only the heaviest projectile loadings, or cartridges which are given anemic propellant charges (designed to remain subsonic), will not break the sound barrier.

    Regardless, the speed of sound in air has nothing to do with terminal performance inits target. The simple calculation for momentum (M x V = P) will show the relative force required to halt progress of a projectile... this force is imparted through the viscosity of the medium which it strikes upon the frontal area of the projectile. The simple calculation for Kinetic Energy (1/2 x M x V^2 = KE) will show the amount of (destructive) work which the projectile carries into the target.

    To compare .45acp and 9mm:

    230gr @ 850fps vs. 115gr @ 1200fps amounts to

    28 ft-lb/s vs. 20ft-lb/s momentum... which means that if both projectiles had the same frontal area, travelling through the same medium, the heavier-slower projectile would penetrate farther. However, since the .45acp's (basic) frontal area is 75% larger than the 9mm, that force is applied over that larger area results in the likelihood of slightly less penetration (lower frontal density, better braking). This variance diminishes when using expanding projectiles which increase the frontal area of both.

    In terms of energy, both projectiles with the above specs will have approximately 370 ft-lb*. Which simply means that the projectile which penetrates farther and/or displaces more of the medium will have done it's work more efficiently. So, the amount of damage done to a target has very little to do with the KE of the cartridge, since it is possible for the projectile to perform that work inefficiently if the design of the projectile and it's delivery speed are out of balance.

    In summary... Just pick one and get good at using it.





    *NOTE: ft-lb is actually not the correct units for energy... it should read ft^2-lb/s^2 so actually... it wouldn't be to far fetched fora box of ammo to list the performance as'calories per serving'.




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    PLUS ONE

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    I am not a ballistics expert, but would have laughed myself to tears had I been present during that "explanation".....

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    *NOTE: ft-lb is actually not the correct units for energy... it should read ft^2-lb/s^2 so actually... it wouldn't be to far fetched for a box of ammo to list the performance as 'calories per serving'. What we think of as 370 "ft-lb" is actually the equivalent of about 33 calories.

    ft-lb is a measure of torque isn't it? It's what one would use to calculate horsepower.


    BTW, this is what I saw as far as the actual equation goes (They use a conversion factor..):

    When using a system of units that is not self-consistent, or if the weight of the bullet is used rather than the mass, a conversion factor must be added.

    For example, to get muzzle energy E in foot-pound force, where v is the velocity of bullet (in feet per second) m is the mass of bullet (in grains) the formula is

    E = m • v^2 / (2 • 32.1739 • 7000).

    Most sporting arms publications within the United States report muzzle energies in foot-pound force, and, when publishing kinetic energy tables for small arms ammunition, use a dimensional constant of 32.163 lbm • ft / lbf • s² rather than the standard acceleration of gravity of 32.1739 ft / s².

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    molonlabetn wrote:
    The speed of sound varies by altitude, varying from 1116fps at sea-level to 1076fps at 10,000ft above sea-level (and decreasing further at higher altitudes).

    Most major pistol cartridgescan and doexceed the speed of sound,measured atthe muzzle of a standard length barrel (~4"). Only the heaviest projectile loadings, or cartridges which are given anemic propellant charges (designed to remain subsonic), will not break the sound barrier.
    Federal's ballistics chart puts muzzle velocity oftheircommon9mm defense load (124gr HydraShok JHP)at 1120fps through a 4" barrel. Yes that's supersonic, but only just; the shock wave will interfere with the bullet and slow it down quickly.You will also not get a supersonic"crack" from the shot; if you compare a PX4's report to that of its carbine relative the CX4 (with a barrel 4x as long as the pistol but a similar internal design)you'll hear the difference. For practical purposes a standard-load 9mm is not considered supersonic from a pistol even if the numbers technically show it to be.

    A similar .45 defense round in 230-gr has a muzzle velocity through a 5" barrelof 900fps, well below supersonic even though it's had an extra inch of acceleration (with 35 kPa of pressure behind it that can do a lot). Federal thus charts the .45, 85% heavier,as having 414 ft-lbs, while the 9mm has 345, so the .45 has 20% more muzzle energy based on catalog data.

    Whether a round is supersonic makes a difference, but it's not a "state change" in its energylike the difference between 211 and 212 degrees Farenheit. Supersonic projectiles are more stable in flight as long as they remain so; a common problem with standard-load .22LR for long-range targets is that by 100yds the round, which exitedsignificantly faster thanMach 1, has slowed to subsonic speeds. In doing so its own shock wave catches up and disrupts the bullet's flight path with a push from behind, affecting accuracy. High-power rounds like .308 stay supersonic for practically their entire effective range, and subsonic rounds or standard loads through a short barrel never exceed the sound barrier, no in either case this transition doesn't happen.

    Supersonic rounds are also more capable of inducing hydrostatic shock; the shock wave of the bullet as it passes through a semi-solid targettends to build on itself much like a sonic boom in air. The shock wave can have sufficient force to disrupt tissue several inches from the wound channel. A handgun bullet is not shaped nor fired at sufficient velocity to achieve this in a fleshy target.

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    That's why I like my Winchester Ranger "Talon" 115gr +P+ JHP's supposedly traveling @ 1335fps. It produces almost no muzzle flash and has very little recoil, in a G17 at least. I can believe this velocity is accurate, as Double Tap has a 115gr Gold Dot pushing 1415fps and ~510lb-ft. But yeah, the 9mm will get well into the supersonic speed range. And these two rounds would hold that speed at any reasonable "personal defense" distance. I feel well armed with the Ranger Talons, lets put it that way. Btw, as a pilot I'm kicking myself right now for not thinking about the aerodynamic effects of the transition back to subsonic flight, lol.

    I also like Federal's LE Hydra-Shok 124gr +P+ (They claim 1170fps out of a 4" barrel, 1120fps @ 25yds, 1080fps @ 50yds) and the HST 124gr +P (They claim 1200fps out of a 4" barrel, 1150fps @ 25yds, 1100fps @ 50yds), but the Hydra-Shok is a daytime only round as it produces an incredibly bright flash.

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    Weak 9mm wrote:
    *NOTE: ft-lb is actually not the correct units for energy... it should read ft^2-lb/s^2 so actually... it wouldn't be to far fetched for a box of ammo to list the performance as 'calories per serving'. What we think of as 370 "ft-lb" is actually the equivalent of about 33 calories.

    ft-lb is a measure of torque isn't it? It's what one would use to calculate horsepower.


    BTW, this is what I saw as far as the actual equation goes (They use a conversion factor..):

    When using a system of units that is not self-consistent, or if the weight of the bullet is used rather than the mass, a conversion factor must be added.

    For example, to get muzzle energy E in foot-pound force, where v is the velocity of bullet (in feet per second) m is the mass of bullet (in grains) the formula is

    E = m • v^2 / (2 • 32.1739 • 7000).

    Most sporting arms publications within the United States report muzzle energies in foot-pound force, and, when publishing kinetic energy tables for small arms ammunition, use a dimensional constant of 32.163 lbm • ft / lbf • s² rather than the standard acceleration of gravity of 32.1739 ft / s².

    Ft-lb is a measure of force over distance (torque, more or less).

    Energy is the accelerationof a force,over a period of time. The use of ft-lb as the units essentially means that the number is simply a coefficient of the energy produced, relative to the gravitational constant of the earth.

    This method of measurement is flawed, since obviously a given projectile at a given velocity will carry the same energy regardless of which planet it is measured relative from. Using the proper units would solve this problem... but for the sake of reality, it is reasonable to compare projectiles, on earth,in this way.

    In Europe, projectile energy is listed in Joules... Which is accurate.Overall, it's a meaningless argument which I thought was more amusing than anything else. I use ft-lb to describe ballistics all the time, just for the sake of simplicity.


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    Yeah, I've taken quite a few physics courses, and I agree that it is an odd way of representing what they want to represent. Joules, as you say, would be an actual unit of energy, which makes much more sense. Oddly enough I also noticed that at relatively low energy levels, there isn't a huge difference in the actual number you get in Joules or in "lb-ft."

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    molonlabetn wrote:
    Ft-lb is a measure of force over distance (torque, more or less).
    Not exactly, though they are related. Torque is a force modifier and thus a measure of force,measured in pound-feet (to avoid this confusion as much as possible :quirky);more specifically itis the forceexerted by a twisting motion (in pounds)versus the distancealong a radius at which the force is measured (in feet), and the symbol is lb/ft (the metric equivalent is N/m). It translatesthe force of rotation into a linear context allowing the solution of problems involving leverage.

    A foot-pound, whichI believe isequivalent to a Joule (the man Joule who figured this all out originally recorded the relation between foot-pounds and calories),isa measure of energy or work (asdE = W);it is the change in energy in a lossless system equivalent to a one-pound weight being lifted 1 foot against the force of gravity, ft*lb. The relation of force and distance is common but opposite between the two measures. It can also be expressed as force over time;a force in NewtonsF = Fg + x applied for y seconds is F*y Newton-seconds, and its subcomponent x*y can be converted to Joules of workby measuring the forceapplied, the constant resistingforce overcome (friction, gravity, etc) and the distance the object on which the force was applied has moved as a result.

    So, a thought experiment. Place a firearm pointing directly upward in a rig that measures force in pounds, and tare out the rig's weight from the measurement. Load the gun and fire a bullet directly upward. Measure the height the bullet reaches, how long it took for gravity to arrest its upward climb,andeither peak force or if you're really clever the impulse (force over time)imparted into themeasuring device (which is equal to the force impartedon the bullet).The force imparted =F = ma. Knowing the mass of the bullet allows you to find the muzzle velocity v, which is equal to a because the change in velocityis from at rest to its current velocity, a = v-0. That velocityis affected bya negative a causedby Fguntil v again = 0. Measure the distance and mass of the bullet and you can calculate foot-pounds of work done by the instantaneous force of the bullet which is muzzle energy.

    Now, far smarter men than I can devise anapparatus that allows them to calculate thiswithout needing the Empire State Building to measure maximum altitude. We have ballistic gel which exerts a measureable force of resistance to a given bullet that can be used in lieu of gravity itself. First attach a bullet to a rod with a scale on itand measure force of resistance in pounds as you push a bullet through the gel. Fire a bullet and measure distance in feet. We know acceleration of gravity; 9.8m/s2, I think that works out to something like 32ft/s2. That times the mass of the bullet is the force gravity would have on the bullet.Apply the ratio between measured force of resistance and force of gravity and apply it to the distance traveled and you'd have the altitude reached. Multiply by the mass of the bullet in pounds (fractions of a pound actually)and voila, foot-pounds of muzzle energy. In fact, if schools weren't so gun-unfriendly, it'd make a pretty good field trip slash physics experiment.

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    I had a guy tell me once that a .45 ACP had some almost mystical power, so that if you shot someone with one with one, it knocked them down, no mater where you hit them.

    To illustrate his point he told a story of a guy who was being threatened by this giant of a man. The victim shot at this giant with his .45 and the big guy went down.

    the went over to the downed assailant and found him unconscious. the only mark on him was where the .45 had nicked his finger.



    He had a straight face when he told me this and he really believed it. :what:



    Tarzan

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    Oh wow, lol.

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    tarzan1888 wrote:
    I had a guy tell me once that a .45 ACP had some almost mystical power, so that if you shot someone with one with one, it knocked them down, no mater where you hit them.

    To illustrate his point he told a story of a guy who was being threatened by this giant of a man. The victim shot at this giant with his .45 and the big guy went down.

    the went over to the downed assailant and found him unconscious. the only mark on him was where the .45 had nicked his finger.



    He had a straight face when he told me this and he really believed it. :what:



    Tarzan
    Was the big guy picking his nose at the time?

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    Ok then if I shoot someone twice witha 22that would be the equivalent of a 44.:celebrate

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    molonlabetn wrote:
    tarzan1888 wrote:
    I had a guy tell me once that a .45 ACP had some almost mystical power, so that if you shot someone with one with one, it knocked them down, no mater where you hit them.

    To illustrate his point he told a story of a guy who was being threatened by this giant of a man. The victim shot at this giant with his .45 and the big guy went down.

    the went over to the downed assailant and found him unconscious. the only mark on him was where the .45 had nicked his finger.



    He had a straight face when he told me this and he really believed it. :what:



    Tarzan
    Was the big guy picking his nose at the time?

    He is one of those guys who knows nothing about guns, but thinks he is an expert on everything. I have learned from experience with him to just listen and keep my personal opinions on his obvious intellectual deficiencies to myself.

    I am a big fan of the .45 ACP myself, but come on this is so far from reality that it is beyond funny. You canuse big and slow OR small and fast to just as easily accomplish the same task.

    Tarzan


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    Ok then if I shoot someone twice witha 22that would be the equivalent of a 44.
    but the dia of .44spl/.44mag is .429 :P
    "The Second Amendment similarly appears to contain an express limitation on the government's authority. If the Second Amendment is read to confer a personal right to 'keep and bear arms,' a colorable argument exists that the Federal Government's regulatory scheme, at least as it pertains to possession of firearms, runs afoul of that amendment's protections" (U.S. v. Printz, 1997) ~Clarence Thomas (SPJ)

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    Agent19 wrote:
    Ok then if I shoot someone twice witha 22that would be the equivalent of a 44.
    but the dia of .44spl/.44mag is .429 :P
    And on top of that, area of a circle is found by A=(pi)r2. Double the radius and you get roughly four times the cavity size.

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    I had a guy tell me once that a .45 ACP had some almost mystical power, so that if you shot someone with one with one, it knocked them down, no mater where you hit them.

    To illustrate his point he told a story of a guy who was being threatened by this giant of a man. The victim shot at this giant with his .45 and the big guy went down.

    the went over to the downed assailant and found him unconscious. the only mark on him was where the .45 had nicked his finger.



    He had a straight face when he told me this and he really believed it. :what:



    Tarzan

    Well I suppose it's possible that the guy fainted when he relized he was getting shot at, but I would think that such a possibility would be a bit hard to generalize.

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    molonlabetn wrote:
    *NOTE: ft-lb is actually not the correct units for energy... it should read ft^2-lb/s^2 so actually... it wouldn't be to far fetched fora box of ammo to list the performance as'calories per serving'.


    The ft-lb is a correct unit of energy. A pound is a unit of force, not mass (it's a slug-foot per second squared, in the non-metric physics world, the "slug" being the relatively obscure unit of mass in the imperial/avoirdupois/pre-metric system). A force acting over a distance gives units of energy, the foot-pound, in this case.

    -ljp

  25. #25
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    In physics and engineering I recall having to make the distinction between pound force (lbf) and pound mass (lbm). I do recall now that I've been looking things back up in my books that as long as you're consistent, you could choose either one. Depending upon what you want to do, one will be an easier choice than the other and will reduce the amount of calculation that needs to be done.

    If you say that what you "weigh" when you get on the scale is your pound "force" (lbf), then you simply have to make sure that when you do something like F = m*a using lbf, you remove the effect that gravity has on your "weight" to make it a mass. So you must write it as F = m*a/gc, where gc is apparently ~37.2 lb*ft/(lbf*s^2) (this is obviously only if you make m a lbf).

    If you say that what you "weigh" when you get on the scale is you pound "mass" (lbm), then you do not need to take into account the effect of gravity on your "weight" (Because you're saying it's a mass) when doing the calculation F = m*a. But when describing the lbf on Earth, you would need to multiply the "weight" (Once again, a mass) times the acceleration due to Earth's gravity to get the lbf on Earth.

    I recall the former method being better for when I was doing statics problems in a MAE course, because you can simply assume the weight is the actual force and not do any additional steps. It makes sense that you'd want to use the latter in physics, where acceleration is highly variable depending upon where you are in space. We often had to write programs modeling the interaction of two or more moving objects in space, where the gravitational effect they had was constantly changing. The gc used on Earth would have no purpose there, only serving to add extra steps to the calculations the program was repeating every dt to find the "updated" gravitational force after every time interval (The objects were of course, changing position relative to one another over time).

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