One or many, the one AR pistol that any shooter should have if there was such a thing as a "requirement" is a 9" or so 300BO suppressed. For the longest I resisted, thinking this caliber and AR pistols in general were a fad and relatively useless. After shooting a friend's 300BO with my suppressor, I've totally changed my assessment on the pistol concept and much more so on the 300BO pistol. With the appropriate length barrel, 300BO suppressed is stupid quiet and extremely accurate in the 100-yard envelope, and the whole package is quite compact. Obviously we all know this is exactly why the 300BO was developed, and it shows.
My 11.5" 5.56 pistol still retains some pretty decent ballistics, and it has been surprisingly accurate.