Have you read the instructions? We know... But please read the instructions for helpful videos and written steps for installation and trigger modification.

Have you installed the buffer tube? Full-Size and Reverse Stretch MP5’s will most likely need to be short stroked to maintain compatibility with the AR15 FCG. This is due to the hammer having a pivot point that is closer to the firing pin than the MP5’s original FCG, therefore is shorter. The shorter hammer presents the possibility of the bolt carrier traveling past the hammer, and locking the bolt carrier behind the hammer. The silicone buffer provides a soft bolt travel limit. The buffer is not needed in MP5K’s since they have less bolt travel.

Have you installed the shelf insert? Semi-Shelf LS5's come with 2 different shelf inserts to fine tune the fitment of the LS5 to the MP5 or MP5K receiver.

Ejection issues? After significant testing we found that replacing the bolt in some MP5's helps to resolve ejection issues. We suggest the RCM MP5-E. 

Smooth cycling issues? Some hammers need a 27-30 degree chamfer on the top front of the hammer. This will allow the bolt carrier to cycle smoothly on top of the hammer.

FAQ

What LS5 variant do I need for my HK SP5?
You'll need the Full-Size Semi-Shelf LS5 lower receiver for your HK SP5. If you are curious about other variants and compatible MP5s, Click Here

Do your trigger groups include a buffer, or do I need to buy it separately?
Our Full-Size LS5 trigger groups include a durable, high-temperature silicone buffer, so no separate purchase is necessary.

Can I use an AR-15 FRT trigger with your lower receivers?
No, our lowers are generally not compatible with standard AR-15 FRT triggers due to differences in internal geometry and safety mechanisms.

Are modifications required for Geissele triggers when used with your LS lower receivers?
Yes, slight chamfering (approximately 27-30 degrees) on the hammer is necessary for optimal fit and function. Our Trigger Group options come with the Geissele trigger already modified. 

How can I order replacement parts, such as buffers or shelf inserts?
For replacement parts, Click Here

Can I use a Hoffman Super Safety with an LS5?
Yes! However fine tuning may be required with the added complexity.  Ensure that the slip trip cycles freely in the receiver, and the bolt carrier rides smoothly over the hammer.

Does the LS5 Trigger Group fit to HK53 or HK91 style guns?
Almost, but no. The LS5 uses a proprietary ejector lever.  223 and 308 ejector levers are designed and expected to be available by beginning of Q3 2025.

Which grip should I buy?
Check out our detailed description page on our selection of grips here.