The Model 19-15 is intended for use with mil-spec pistol caliber uppers utilizing a straight blowback (non-locked breech) bolt. It is not compatible with CMMG rotary-delayed blowback systems. 

The best function is achieved by use of bolts that do not have the small protruding dingus (It’s a technical term. Don’t look it up) illustrated below:

 

Bolts with the dingus will do in a pinch, but have occasionally induced some minor feed issues in our testing, while those without have proven to be 100% reliable.