American Precision Arms G4L5860S Gen 4 Little Bastard Self Timing Stainless 5/8"x24 Threads 2.70" OAL 1" Diameter 6mmThe new calibrated U. R. P tunable top ports have been designed for maximum adjustment making the Gen 4 line of brakes the flattest shooting brakes available hands down. By testing port size, location, and how the gas feeds into them, we have been able to make these tunable top ports capable of generating zero vertical movement.