Yeah, there's a whole slew of leftover black-powder-era cartridges still in use (in the US at least) which have shells "too big" for what you get.
It keeps the peak pressure down though...easier on both the gun and you.
Think of it this way: say you've got a load for the old 45LC (dates to 1873) involving a 230grain JHP @ 850fps, and another for the 45ACP that nets you the very same external ballistics - same bullet, same speed. The 45LC will get there with something around 15,000psi (roughly - I'd have to look up a loading manual and it will vary by powder chosen) while the 45ACP will need about 21,000psi.
In each case that's "peak pressure". The old LC will hold it's pressure at peak for longer. Graph it out on a curve, the ACP will "spike" more sharply while the LC's curve will look shorter but broader.
This directly affects how the gun feels in your hand. The LC will feel like a "big steady push" where the ACP will have a bit more of a "sharp crack".
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For this same reason, one caliber I've got a lot of interest in is something called the "356GNR". It's a wildcat - you take basically any 357Mag gun and alter just the cylinder. The shells are 41Magnum necked down to 357. If you use it to get 357Magnum ballistics, you can do it with a lot less peak pressure, OR you can hot-rod it to where 158s are pushing past 1,800fps, at least in a strong gun (Ruger Blackhawk).
The way to get there cheap is to score a 357Mag/9mm factory convertible and send the 9mm cylinder off to Gary Reeder or one of the other gunsmiths with a 356GNR chamber reamer. You can still shoot 357Mag/38spl in the "normal" cylinder.
Case capacity matters
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