Bolt gap is the measurement that decides whether a roller-delayed gun runs. JP calls it the most critical parts relationship in the JP-5, and it drifts as the rollers and locking surfaces wear. This two-piece go/no-go set replaces the feeler-gauge guesswork with a quick in-or-out check.
The use case is simple: the gun starts short-stroking or throwing brass differently, the shooter checks bolt gap first. If the gap has walked out of range, JP’s fix is a different roller size, which is why the gauges and the roller kits belong in the same range bag.
One thing to know: JP does not publish the two gauge thicknesses on its product page, so think of the set as JP’s pass/fail window for the JP-5 rather than a general-purpose gauge set. It will check an H&K MP5 the same way, since the mechanism is the same, but the window is JP’s spec.
The trade-off is that a $30 tool only matters if the shooter is willing to open the gun and measure. For anyone who shoots a JP-5 in volume, it is the cheapest diagnostic they will buy.
