What purpose? Fun question. When one is trying to suppress the noise made by firing a weapon, killing or reducing, the muzzle blast is only about 1/3 or the problem. If the bullet travels at supersonic speed then the sonic boom is also very audible. If one eliminates all the noise from the muzzle that leaves the “slap” of the action as it cycles in an auto loader. Locking the breech during firing eliminates the mechanical noise from cycling.
Bolt actions, on the other hand, only make a small bit of noise when one works the bolt and even that can be tuned down to zero with a little work.
It takes away some of the muzzle energy when a round is fired in an auto loader since cycling the action can take away as much as 1/4 of the bullet’s muzzle energy. So locking the breech for firing can dramatically improve range and energy at impact.
For covert type purposes one shot may be all that is needed/available so maximizing energy and minimizing noise are top consideration. If one needs rapid fire then none of that matters from the second shot on.
FWIW I make a twist lock breech lock conversion for Ruger MK 1,2,3, 22 cal pistols. That’s some quiet stuff right thar.
SO I whistled right past the obvious. Sheesh.
Several handgun cartridges are subsonic - 45 ACP, 38 SPL, .380 Auto (not the +P versions of these which have more zing to them). A 40 cal with a heavier 180 gr bullet would be subsonic in most cases, though there are some really stout brands of ammo out there.
Sorry - rackin’ the ammo slide.