Ancona is right, though so is that saying "beware the man who shoots just one gun" - as someone who shoots many (gunsmith and competition) it's really true unless you get a
lot of practice.
Not sure I'd agree that the .223 is wimpy compared to 7.62x39. Those really lose their ambition past 100 yards, and most shoot pie plate groups at that range - we call them minute of milk jug here. .223 is nice out to around 400 if you know your holds, nastier on target too. The 7.52 shines in very close quarters mainly. I'll take my .223s with their <1/4" groups over a 6" group out of an AK or SKS any day. If you don't hit 'em, it doesn't matter what you missed with!
Nato 5.56 chambers are spec'd slightly larger than .223 civvie to work with bent and sloppy and dirty mil ammo - and some of that is pretty bad stuff, which is why it's "cheaper than dirt"
But both guns and ammo vary, and all ammo is originally SAAMI spec'd to the smaller .223 standard, including the nato stuff - the mil chamber spec just allows for more tolerance slippage. And damage between manufacture and actual use.
FWIW, the NATO chambered chrome lined .223's are the
least accurate available due to the sloppy fit and the lousy plating process (it's hard enough to get right on a flat plate, now inside a bore tube...not so easy.
I will say that in slow and medium speed competition type venues - switching from a .38 target gun to a .45 doesn't give me any trouble at all these days, but both are large heavy guns I get a lot of practice with. EG the revolver is a S&W mod 14 with a 6" barrel,
not my puny carry gun, which I have to re-learn every time and force myself to practice with as it's not fun to shoot (glad for that laser on it). So, not that different - the S&W .38 is a much better trigger and easier to recover from recoil, that's it - so in fast shooting, I'd rather have it unless I really needed a ton of bullets right now (I do use speedloader carriers for the revolvers so that's nearly a wash).
I'd have a lot more trouble going from my CZ to a Glock, for example as grip is critical and they are shaped different - my muscle memory would work against me and does.
The only reason I can think of to need a real buncha bullets in one encounter is if I'm outnumbered many to one - and that's simply a situation you have to avoid, because movies notwithstanding, you're going to lose. Shoot and run away, live to shoot another day.
As the marines say - anything worth shooting is worth a double tap, but more is waste. They also say - if you find yourself in a fair fight - shoot your planner. You won't need to if you're the planner and in that fight, someone else will handle that one for you - so
do think about that when sweating how much firepower you really need - it might be a waste of weight and size. Even the DHS (and they can't shoot for crap - we teach them for re-qual here sometimes) only ordered about 1.5 bullets per person in the country.
As Col Plaster says, too easy leads to half-assism. Every shot is the first shot of the rest of your life is a better approach, then having a zillion doesn't seem as important -
the first one ended the fight - if it's a hit. I don't care what you have - if you have 10 against one, no weapon can force them all to keep their heads down
at once - you're toast because the one you missed won't miss you. A minigun wouldn't save you if there was just one guy off to the side or behind while you get distracted with a main force.
Therefore, the other part of that equation is having that association with like minded people you can count on in a pinch. It might seem chicken to run and hide with them until you can venture forth in force - but I'll vote for live chicken over dead duck every time.