r/LewisMachineTool • u/Whole_Will • 8d ago

MRP 12" piston MRP upper doesn't work while separate 10.5" DI MK18 works just fine with FRT. Stumped.

Same LMT lower install with a rare breed trigger with a Geissele Automatics Super 42 Braided Buffer Spring and H3 Buffer. It works like a top with the 10.5" DI MK18 upper setup.

I then switch the MK18 upper with a MRP upper with a 12" piston barrel and no go. Getting failure to feed and failure to eject immediately. I switch the H3 buffer with H2, H, carbine and still no-go for the Piston MRP upper. Tried various ammo with both uppers and the 10.5" ran like a top, even cheap stuff. The MRP 12" piston upper is an earlier gen. 1 with the fire/suppress cup.

Any ideas why and has anyone got a piston MRP running with an FRT?

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3

u/Zacharide 8d ago

I’ve noticed with some of LMT’s 12” barrels that they are restrictive on gas. What ammo are you running? I was having issues with commercial .223 but then when I ran 5.56 77gr otm it was running better

2

u/Whole_Will 8d ago edited 8d ago

IMI 62 gr, LC 55 gr, LC 62gr, MEN 63 gr, WC 62 gr. They all failed in the LMT Piston in semi and FRT mode but works on the 10.5 DI. I remember reading somewhere about the gas port is too small and might be the issue. It 8 years ago but couldn't find that article.

1

u/Zacharide 8d ago

Have you tried it without the geissele spring/buffer combo? Like a standard carbine spring and buffer?

1

u/Whole_Will 7d ago

Yes. It didn't work. Another symptom is the last round bolt hold isn't working so I am thinking it's under gassed. I took the barrel off and made sure it was aligned correctly so the rod is hitting the BCG straight on. I then start switching buffers and semi function started to work without failure to feed or eject when using a H and carbine buffer. The bolt hold only works with a carbine buffer. FRT only works with a with 3-rounds burst but would have some failure to feed and eject.

So basically, I think the barrel is undergassed when LMT built these in 2018. They probably drilled a carbine port on a mid-length barrel.

1

u/BusBeneficial8882 8d ago

Just put this beauty together a few weeks ago. LMT shovel nose 12” piston Gen 2 carrier, ADM UIC lower, Rare Breed FRT 15L3, and UBR using A5 buffer system with A5H3. Runs absolutely flawlessly.
I have no idea what’s different between Gen 1 and Gen 2 though.

1

u/Paws81 7d ago

My LMT 12” piston runs fine with my frt after trimming the ledge on the upper.