These other (not X-products) launchers are a replacement muzzle device for a threaded barrel, so barrel length is a non-issue (assuming that this threaded barrel is legal in your state).
Or 308 if it's a new production in from a combloc turned NATO allie.
Pk is the model,
pkm is one of the improved variants . Like most stamped Ak's in 7.62 are really akm's vs ak47.
But it's common for most to just call all pkm's .
Pk improved = pkm
pkp is a lightened version with fixed barrel , also a rare bullpup based on it .
pkt vechical mounted remote fired .
Other variations are different mounts or night sights.
Check out the X-Products facebook page, they're releasing a redesigned Can Cannon with a blank-only chamber that will address BATFE's concerns and make it a non-NFA item.