Arizona Fusilier said:
Is there some efficiency or performance degradation as one moves up or down with their caliber choice?
Generally, yes. Usually a can designed for a smaller caliber will be quieter with that caliber than a bigger can will be with that same caliber.
It's true that cans for larger calibers are generally larger, and usually larger cans tend to be quieter than smaller cans. But this is usually counteracted by the fact that they have a larger bore size and therefore let more gas escape quickly. For example, a 9mm can shooting 9mm will almost always be quieter than the equivalent .45 can shooting 9mm. The same goes for most rifle calibers: A 7.62 SDN-6 is a little bit louder with 5.56 than the M4-2000 (its 5.56 equivalent) is with 5.56.
But this isn't always the case; sometimes if there's a big enough jump in size the larger bore size won't overcome it. For example, my Octane 9 is quieter with .22 than the Spectre II is, even though it's bored out to 9mm instead of .22. This is probably because the 9mm can is big enough that the huge jump in internal volume makes up for the fact that the bore is so much bigger.
Another exception is the 7.62 Saker, and that's because you can change out the endcap on it. So if you put the 5.56 endcap on the 7.62 Saker it will be quieter with 5.56 than the 5.56 Saker is.
Arizona Fusilier said:
May I presume the same principle holds true for pistol silencers?
Yes. Though there's one glaring exemption for both rifle and pistol silencers: You usually don't want to shoot .22 though it, even though it's safe to do so. That's because .22 is a very dirty round that leads up the silencers, and most rifle cans aren't designed to be taken apart and cleaned because with centerfire rifle ammo you don't need to clean it. Most pistol cans are designed to be taken apart, but the lead from shooting .22 will make it much more difficult to take apart. Most .22 cans today are not only designed to be taken apart and cleaned, but they're designed to be taken apart and cleaned even with lead gumming up the internals. Note that I mentioned shooting .22 though my Octane 9 pistol can: The Octane is an exception because it has the same baffle design as the .22 Spectre II and therefore is still easy to take apart when leaded up.
Arizona Fusilier said:
I always thought this stuff had to be "exactly so" if for no other reason than safety.
Yes, but it only has to be "exactly so" in one direction. It's actually far safer to shoot 5.56 through a .308 can than it is to shoot .308 thought it: 5.56 will stress the can less, and the smaller bullet diameter means that a small misalignment of the can is much less likely to cause baffle strikes.