13.5" barrel with the standard Sig birdcage (Keymod Gen II carbine). Only mags that work are Sig Gen 2's. Two 30's assembled end to end hold 70+ rounds - never have to do a mag change in PCC match. First thing to do is replace Sig's FCG - I went with HyperFire 24C - after 3,500+ rounds, it started doubling - hammer/sear interface chewed up - Hyperfire gave me a new one- have about 1,500 rounds thru it. I reload using 115 gr XTreme Heavy Copper Plate. 4.7 grs. WSF ans S&B primers. Chrony at 1190 fps. Zeroed at 25 yds. 1 group at 25, 2 at 50, 3 at 75, and 4" at 100 yds. with no problem (YEMV). Have to use a firm hand seating the mag with the bolt closed. Soft shooting - no issues.