Putting an FRT in all my LMT Rifles!

Published on January 14, 2026
Duration: 10:15

This guide details the process of modifying AR-10 platform lower receivers, specifically LMT MARS-H and KAC SR-25, to accommodate an AS Designs Arcfire FRT. The host, demonstrating high mechanical expertise, explains that the internal shelf of these high-shelf lowers requires milling to provide adequate clearance for the FRT lever, a crucial step for reliable function. The video contrasts this with the easier integration of FRTs in 5.56 platforms like the LMT MARS-L.

Quick Summary

Modifying AR-10 lower receivers like the LMT MARS-H and KAC SR-25 for an AS Designs Arcfire FRT involves milling the internal shelf to provide clearance for the trigger lever. This is necessary due to the distinct internal geometry of AR-10 platforms compared to AR-15s, preventing binding and ensuring reliable cycling.

Chapters

  1. 00:00Introduction & Shooting Montage
  2. 01:24AR-10 FRT Tuning Challenges
  3. 02:11Technical Failures Explained
  4. 03:41Lower Receiver Modifications
  5. 06:09LMT 5.56 Performance
  6. 07:59Spuhr BAM Mount Overview
  7. 08:34Final Testing & Conclusion

Frequently Asked Questions

What modifications are needed for an AR-10 lower receiver to use a Forced Reset Trigger (FRT)?

AR-10 platforms with high-shelf lower receivers, such as the LMT MARS-H and Knight's Armament SR-25, require the internal shelf to be milled or 'clearanced'. This modification ensures the FRT lever has sufficient movement without obstruction, which is essential for reliable function.

Why are AR-10 platforms more difficult to run with FRTs than AR-15s?

AR-10 platforms have different internal geometries compared to AR-15s, particularly regarding the lower receiver's shelf height. This difference often necessitates milling the shelf to accommodate the FRT lever's full range of motion, a step usually not required on standard AR-15 lowers.

What issues can arise from installing an FRT without modifying an AR-10 lower receiver?

Without proper modification, the FRT lever can bind in the rearward position, or the bolt carrier group may catch on the FRT lever, leading to cycling failures and malfunctions. These issues stem from the AR-10's distinct internal dimensions.

Which specific AR-10 lower receivers were discussed as needing modification for FRTs?

The video specifically mentions the Knight's Armament (KAC) SR-25 and LMT Defense MARS-H as AR-10 platform lower receivers that require internal shelf milling to properly function with an AS Designs Arcfire FRT.

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