It you can't afford or can't be bothered to reload, (and have a better centerfire), then just stick to the .22lr. over 90% of the time, you need not hit anyone in order to make an attack stop, and over 80% of the time, you need not even fire. VERY few have the determination, skill and proper gear for mere LUCK to not be the main determinant of the outcome of an attack, (if the "bluff" of just seeing the gun, or hearing MISSES fired, is not enough).
I know nobody likes to hear the above, or likes to "only" have a .22r, but being TRULY skilled with a .22 is better than being slow/inaccurate with a .45, or 357, and beats the hell out of having a shotgun that you can't GET TO in time! :-)
If you can't toss up a pair of soda cans with the hand that "clears" your concealing garment, draw an Airsoft gun and hit both of the cans in midair, most of the time, you are wasting time and money gonig to the range, even if 'all" you fire there is .22lr. Get busy in your back yard, garage, or basement with the Airsoft. they cost about 1/2 c per shot, so what is your excuse? If you are not fast and accurate with airsoft, I GUARANTEE that you are slow/inaccurate with the .22, and if you can't "do it" with the .22, (any given exercise of relevant speed and accuracty) then you CERTAINLY can't do it with the centerfire.