The lock piece is the JP-5’s tuning knob. JP compares it to an adjustable gas block on an AR: swap the angle and the recoil impulse changes to match the ammo. This is the 55 degree piece, one of the lower angles JP offers, which means more delay and a slower-opening action than the 80 degree piece a JP-5 ships with.
JP’s rule is that a larger angle opens easier. Weak or unreliable cycling calls for a higher angle; a gun that cycles violently and slams the buffer tube calls for a lower one. JP lists 80 as the standard and notes that competition shooters often drop to 70 for recoil reduction with standard-pressure loads. Going down to 65 and below is for hotter loads or a shooter chasing a flatter gun who is prepared to test.
One thing to know, straight from JP: under-delaying the JP-5 with high-pressure ammo can damage the gun. A 55 degree piece adds delay, so the failure mode is the other way, weak cycling with soft ammo. Either way, a lock piece change can shift bolt gap, so measure it after the swap. JP lists these as compatible with the H&K MP5 and with clones that accept H&K lock pieces.
The trade-off is that $100 buys a tuning step, not a guaranteed improvement. A shooter who does not know their current angle and ammo velocity should start there before buying.
