Hop-Up Arm

Adjusted by the gun's hop-up dial, the arm translates a turn of the dial into physical pressure on the bucking, which in turn grips and spins the passing BB. Its width, contact shape and how evenly it seats matter as much as how far it's dialed in.

A worn, chipped or poorly seated hop-up arm applies uneven pressure across the BB's contact patch, which curves shots left or right inconsistently even at a correct overall hop setting — a common cause of a gun that groups poorly despite an otherwise sound barrel and bucking.

Replacing a worn arm is a cheap, quick fix compared to diagnosing everything else that could cause curving shots, so it's often one of the first parts technicians swap when troubleshooting a gun that groups worse than its barrel and bucking suggest it should.