LMT MRP barrel and upper cut down questions

AR/M4 AR Technical Discussion
1

So, I’ve tried searching, but I haven’t found the answers to these specific questions. Any and all help greatly appreciated!

1. Cut down LMT factory MRP barrel from 14.5" to 10.5"
If I do this, will I need to adjust the gas port size, and/or switch to an adjustable gas block, or will it run as-is? Any other concerns of which I should be aware? Assume mostly quality 55gr ammo for plinking, hand loaded heavier stuff down the road. 95% unsuppressed, running with an A5 receiver/buffer system and stock BCM bolt carrier group.

2. Cut down LMT factory MRP 300blk barrel from 16" to 10.5"
Again, if I do this, will I need to adjust the gas port size, and/or switch to an adjustable gas block, or will it run as-is? Any other concerns of which I should be aware? Assume mostly quality supersonic (125-150gr) ammo for plinking, hand loaded 125-150 supersonics down the road. 100% unsuppressed, running with an A5 receiver/buffer system and stock BCM bolt carrier group.

3. Cut down LMT MRP rifle length upper to CQB length
I have two of these rifle length uppers sitting around (with 16" barrels), and would use them on SBRs (with different barrels) if they were the shorter CQB rail length. Is this even possible? If so, can anyone do it, or should I go to an acknowledged specialist. Is it a stupid idea? I could just sell them as used uppers and buy more CQBs, but maybe this would be an easier way to go? Or not?

Thanks in advance for your advice!

2

  1. Depends on how many rounds through it you have and if you plan to run surpressed. I personally would not adjust the gas diameter as it’s likely a little over gassed.

  2. Can’t answer this.

  3. I would trade someone or sell it outright. These still go for 600 (upper only) all day long without much trouble. Cutting it I feel is going to be a disaster and you will lose your sling attachments at the front end.

3

Thanks for the responses. For sake of clarity, both of the barrels for questions 1 and 2 above are brand new, never been mounted in an upper or fired.

4

I second the idea that cutting the upper isn’t the best idea. Sell the chassis or trade for what you want. I’d be interested in buying or trading for a rifle length chassis if you go that route!

5

I wouldn’t cut the upper. MRP uppers are scarce these days (I’m sure LMT’s busy fulfilling the NZDF contract) and you can probably sell it for a good price, especially the longer-railed MRPs, which aren’t as common in the first place.

6

Thanks for the advice all. Selling off the MRP barrels, and will be shopping for CQB upper receivers. Krichbaum - please send me a PM if you have a MRP CQB and want to trade for one of my rifle length ones…

BTW, LMT said cutting down the barrels would “void the warranty”. I’ve never made a warranty claim on anything firearm related, much less a barrel. I wonder if this is actually a “thing”…

7

I was just about to post this thread with a few different stipulations! (Not to hijack)

I have a CQBP12" SBR in nfa jail right now I’m not so sure I want to wait for it and am considering selling. I can stand not having a can for 10 mos but a rifle? UGh! So here’s what I’m thinking:

  1. Finding a MRP 14.5" rifle (DI or GP) with SF FH. Used I’m guessing because I don’t know if LMT offers a 14.5" option.
  2. Finding a 16" MRP rifle (DI or GP) and cutting it to 14.5 and pinning a FH.
    -----How hard is it to remove a LMT Gas Block? And where would I send the barrel for the work and what will the $$ be?

Thanks guys for any help.

8

Sent you a message.

It’s pretty standard that cutting down a barrel will void the warranty. They can’t cover something when it’s being modified to that extent. But if you have a good barrel and have someone that knows what they’re doing, it will turn out fine and work great.

9

Message received - thanks krichbaum.

I contacted Jarod at Parker Mountain Machine and Marvin Pitts/Nefarious Arms - both said they could do the chops to 10.5", but LMT wouldn’t give me any info about what I might want to do with the gas port sizes, so I figured I’d just divest and repurchase the OEM 10.5" barrels. The two barrels mentioned in the original post (and one other in 6.8SPC) are listed on the EE here: https://www.m4carbine.net/showthread.php?200794-LMT-MRP-barrel-assemblies-(and-1-BCG)-5-56-300blk-and-6-8SPC for anyone interested.

I answered some/part of hypno02’s question via PM, but for those seeking the same info: yes, LMT makes and sells an MRP 14.5" 5.56 DI barrel assembly. I haven’t seen a complete rifle/carbine available with this barrel, but the complete uppers and loose barrels are definitely available in that length. Jarod said he wasn’t sure if he would need to remove the gas block to do the chop.