JP-5 Lock Pieces

Selecting a Lock Piece For Your JP-5™

Lock pieces for the JP-5™/MP5 pattern operating system are distinguished by the angle of the roller bearing surfaces. Most JP-5™ models will ship with the 80° lock piece. This angle determines how much force is required to open the action. The larger the angle, the easier it is for the action to open. So, systems with higher pressure will take a lower angle of lock piece and vice versa.

If you increase the pressure in your system—e.g., changing to a longer barrel, adding a suppressor, switching to higher pressure ammo—a lower-angled lock piece will function better. If you are decreasing the pressure, a higher-angle lock piece might function better.

In general, if your JP-5™ cycles weakly and unreliably, a higher angle of lock piece is recommended. If the bolt is cycling violently with the SCS bottoming out noticeably in the buffer tube, a lower angle of lock piece will help. Be aware that under-delaying the JP-5™ combined with high-pressure ammo can damage the gun.

In our standard 14.5" barrels, we have found the 80° piece to be the best all-around angle to reliably run low-pressure ammo such as Steel Challenge hand loads or the 130 Federal Syntech while also reducing recoil for high-pressure loads.

Competitive shooters looking to optimize for recoil reduction will opt to move down to a lower angle, typically the 70°. The 70° piece will run any standard pressure loads but will be closer to the edge of the operational window with low pressure loads like the 130 or 150 Federal Syntech or similar low-pressure hand loads. When running the 70° lock piece with such loads, we recommend more regular cleanings of your JP-5™. Several of our top shooters are running this lock piece successfully.

Compatibility

JP-5™ lock pieces are compatible with the H&K MP5 platform but not all MP5 clones. Any non-H&K MP5 clone that takes H&K lock pieces will be compatible with JP-5™ lock pieces.

Because of the precise geometry of the locking surfaces, switching to a new lock piece may alter the bolt gap of a given rifle. Because of this, we recommend validating bolt gap of any rifle after changing out the lock piece.