As far as I remember (not touched GRB for ages since I'm on Aux'ing the other CM weapons), GRB outright destroys SABR in CQC, and up to medium range due to its mobility bonus. For whatever reason, I never got the results I had in CQC with SABR as I've had them with GRB (ie, GRB is always surprisingly good). I don't see the technical reason behind it, though. SABR is for crouched, static fire freaks. And it's a long range weapon, which burst variants are NOT, at all. If you compare long range ability of GRB vs SABR, ofc SABR is coming on top, it's its job to be better at range IMHO you're better off fitting your GRB with ALS and use it as a medium/close option. In fact, a versatile option.