I installed a RB FRT-15 in one of my SBR lowers yesterday, carbine spring with H3 buffer.
I ran one 20 rd mag 5.56 from 10.5", with SilencerCo Omega, through it. Just controlled double shot bursts.
I also own a factory CMMG Guard 8" 9mm pistol, so I moved its upper to the SBR lower with FRT-15.
I ran three endomags filled with 9mm through the guard upper, no extra weight in carrier, with Thompson Machine SG-2. It functioned fine using the same spring and H3 buffer. Two of the mags I was just shooting to get a feel for it. Was successful in getting single shots several times, with the occasional double or triple. Did one 9mm mag dump, functioned properly, this trigger is fun.
A Fostech (with upgraded trigger springs) resides in my CMMG lower, so I'm used to doubles. Doubles from the FRT-15 trigger are faster and more fun.