So, this is all boiling down to market perception is what I am hearing, not physics.
Is the performance gained from firing the .380 from a 4.5" barrel so negligible from a ballistics viewpoint?
There is just so much you can do with a small volume case. There's even less you can do with a case and firearm system designed with a maximum pressure of 16,000PSI. If that's not a physical limitation
, I don't know what is!
Honestly, I bet if you built a Thompson contender barrel around .380, and made it so you could cut the barrel down a quarter inch, shoot & measure velocity then cut again, you would reach a major threshold right around three inches where there is just a little
(like 5-10 fps per half inch) improvement up to 5 inches, and then virtually nothing.
I just checked my load books. Here's the facts: 9mm Luger can use about 30-40% more powder
(for a given powder) for a HEAVIER
bullet (100 gr 380 vs 124 gr 9mm), at a pressure of *DOUBLE* that of 380. We're talking 100 grains at 880 fps vs 124 grains at 1100 fps... Basically double the muzzle energy for a case that's a whopping 3/16" longer, and a little wider. If you count 147 grain or +P defensive loads, that makes it well over 100% more bang.
To answer your question, yes, the performance "gained" is guaranteed to be negligible, due to gas volume alone... 9mm from a 3" barrel is still going to be worlds more powerful than .380 from a 5" barrel.
And you can't make ammo that delivers more pressure, due to fact that people will ignore all warnings, use it and subsequently spontaneously disassemble their more antiquated pistols.