Most AR pistols use a proprietary stubby bolt carrier and an accompanying stub buffer in the upper itself to get the no buffer tube look… Others use a slightly shorter carrier with a short tube/buffer/spring setup(like the one cossey posted a picture of)
Essentially the pistols short stroke and they require fine gas system tuning to function correctly. Some guys have actually been hand loading subsonic .223/5.56 rounds and running suppressed with epic results in that setup 
Is that stock legal on your preban…yes as long as your barrel is at least 16" breech to muzzle which also maintains the 26" min OAL.