About to try my hand in 3gun also.
This is my build. waiting for my surefire brake to come in and need to decide on what scope.
Mega upper and lower
Noveske NSR 16.7 hand guard
18 in JP barrel
JP low mass BCG
AR-15 Silent Captured Spring
SLR DA7-SSC Adjustable Gas Block
Magpul fixed stock
Magpul K2 grip
sense there are no fwd serrations I put an AR gas vent
and I change the shown trigger to a Hiperfire hipertouch 24C