12.5" LMT Piston - Ammo Choices

AR/M4 AR Technical Discussion
1

Does anyone have issues with their 12.5" LMT piston uppers digesting only 5.56 pressure ammo reliably?

I took my brand new 12.5" LMT piston to the range last weekend - with the following results. I started with the provided H2 buffer but only had me re-loads with me.

25gr of H335 / 55gr FMJ Hornady resulted in short stroking / FTF :frowning:

I came back another day with H buffer and more variety of ammo.

  1. XM193 - cycled fine / locked the bolt on last round - no problems

  2. Win White Box 55gr - cycled / did not lock the bolt open

  3. Win White Box 62gr - cycled / did not lock the bolt open

Re-loads

  1. 26gr of H335 - cycled / locked the bolt open (the 25.5 / 25.7 / 25.8 cycled but did not lock the bolt open)

  2. 24.5gr of RL-15 - cycled / locked the bolt open.

I got Car buffer to try next and some Ramshot TAC with the 55gr.

Any comments? Suggestions?

Thanks

Paul

2

I have a carbine and an HSS buffer that I’ve been testing on my MRP 12" piston.

It runs few .223 rounds effectively. Modern commercially loaded .223 (like Remington UMC or Federal value packs) seems to have a better track record than Wolf or even Hornady Training 55gr.

It has run all of the 5.56mm I’ve fed it cleanly.

3

Thats pretty much what I found - 5.56 pressure ammo and hotter re-loads all cycle reliably.

Commercial .223 and mid range re-loads short stroking issues. I haven’t tested the car buffer yet though - only H2 and H.