LS5 Installation Instructions

Click links below for video installation instructions.

Before you begin installation, ensure that the firearm is safe and clear. Please also check to ensure you are using compatible parts. 

Attaching LS5 Trigger Group to MP5/K

  1. Ensure magazine is removed and weapon is safe.
  2. Remove End Cap and original Trigger Group.
  3. Put the Bolt Carrier in the forward most position.
  4. In the LS5, ensure the Hammer is cocked and the front of the Ejector Lever is down.
  5. Pivot the LS5 Trigger Group onto the Receiver.
  6. Slide the black Buffer tube over the Recoil Spring if you have a Full-Size or Reverse Stretch MP5.
  7. Reinstall End Cap and Push-Pin(s).

Assembly of Trigger Housing

  1. Remove the Ejector Lever retaining e-ring from the Ejector Pivot Pin, then remove the Ejector Lever and PTFE shim from housing and set aside.
  2. Place the Trigger and Hammer in the housing, then insert the Safety in its hole.
  3. Insert the Hammer Pins. If the Trigger’s cassette has locking set screws, ensure they’re tight.
  4. Install the Safety Detent and Grip with the Safety Spring.
  5. Test Fire Control Group (FCG) for function; Hammer stays cocked in SAFE, Hammer falls in FIRE, Hammer does not fall on Trigger Reset.
  6. Insert Ejector Pivot Pin from the outside of the housing. On the Inside of the housing, place the PTFE shim over the Pin, followed by Ejector Lever and Retaining E-Ring.

Buffer

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.

Compatible Triggers

Many AR15 and AR10 triggers will work in the LS5 Trigger Housing with modification, and some will work without any modification required. Below is a list of triggers that will drop in:

  • HiperFire EDT line
  • Rise Armament RA-535, Blitz, RA-240, RA-242
  • Ballistic Engineering AR triggers
  • Timney AR15 Competition line
  • ALG ACT
  • Franklin Binary AR

Trigger Compatibly, Fitment & LS5 Installation Details

When getting a trigger to function properly in the LS5 there are two main parameters to meet:

  1. Does the FCG allow proper range of motion for the Ejector Lever?
  2. Does the Bolt Carrier ride smoothly over the AR15 style hammer?

Many drop-in triggers have cassettes that are made of aluminum and can be easily filed to fit under the Ejector Lever.

The hammer can be polished at a 27-30 angle until the Bolt Carrier cycles smoothly.

Inserting the Included Shelf Insert for a Semi-Shelf LS5

Semi-Shelf LS5's come with 2 different shelf inserts to fine tune the fitment of the LS5 to the MP5 or MP5K receiver. The larger of the shelf inserts fits HK models and the smaller of the inserts generally fit PTRs better. 

How do I install a Super Safety in an LS5?
Watch this instructional video

How do I install an Atrius FRS in an LS5?

  1. Install your LS5-ACI into the rear of the trigger pocket.
  2. Insert Atrius Detent Shaft through the right side of the LS5 about halfway into the receiver.
  3. Install the Spring and Cam-Lever before pushing the Detent Shaft all the way through the LS5. Ensure that both the left side switch and the right-side indicator/switch are both in the same firing position.
  4. For video instructions skip to the 1 min. mark in this video by Atrius.

Please refer to the trigger compatibility section above for fitment guidance. We currently recommend the ALG-ACT trigger for your FRT build.  You will also need to install a slip trip to allow the Atrius FRS to function. 

To install a slip trip - 

  1. Position the slip trip so that it securely grabs onto the back of the bolt carrier group (BCG).
  2. Ensure it’s seated evenly and aligned so it moves naturally as the carrier moves forward.
  3. When fully collapsed, the slip trip should sit flush against the back of the BCG.
  4. With the slip trip attached, slide the BCG into the receiver.
  5. The slip trip should be positioned about halfway on the carrier during insertion.
  6. The BCG should slide smoothly into the receiver without resistance or binding.

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Printable Instruction Sheet for Standard LS5 Installs