View Full Version : LMT MIM gas key failure
I have come across numerous discussions of MIM gas keys on LMT BCG's. There seemed to have been some heated debate years ago about whether MIM gas keys were the worst thing ever and a horrible cost cutting measure by LMT, or whether MIM gas keys were "nothing to worry about", with LMT firing thousands of rounds with no failures on any MIM gas key'd rifle.
Anyways, a month or so ago, I opened up my older Defender MRP for cleaning, and the first thing I noticed was that the front of the gas key had "mushroomed", splitting in a few different directions, appearing to have peeled backwards in a few spots (like a blocked barrel on a rifle from old Looney Tunes cartoons). I contacted LMT immediately, and they had me send in the entire upper. Just received it back last night, and they replaced the gas key with a non-MIM one. Notes on the order state they fired 60 rounds of M193 and it functioned fine after gas key replacement. With them having fixed it and tested it, I'm comfortable with the rifle again. However, I thought I would share this with the forum since I read so many reports of people saying MIM gas keys were nothing to be concerned with, as well as stating they had never seen any pictures of a MIM gas key failing. As soon as I get home (in an hour or so) I will post up the pics I took of the BCG that I sent to LMT, so people can see first hand that yes, MIM gas keys can fail, and I'm glad that LMT doesn't use them anymore and replaced it with a new one.
Was it malfunctioning before you noticed it and sent it back?
Interesting you found a deformation like that. All the keys on my new LMT BCGs are non-MIM as you described.
Was it malfunctioning before you noticed it and sent it back?
Nope, was working just fine.
https://i.ibb.co/YRWZ0px/image1.jpg
https://i.ibb.co/dBfwbcm/image2.jpg
https://i.ibb.co/tMRSJ2j/image3.jpg
Work order receipt
https://i.ibb.co/m50tvJ2/IMG-4373.jpg
New gas key:
https://i.ibb.co/bNpj5rQ/IMG-4374.jpg
https://i.ibb.co/YRWZ0px/image1.jpg
https://i.ibb.co/dBfwbcm/image2.jpg
https://i.ibb.co/tMRSJ2j/image3.jpg
Work order receipt
https://i.ibb.co/m50tvJ2/IMG-4373.jpg
New gas key:
https://i.ibb.co/bNpj5rQ/IMG-4374.jpg
I actually had that, and worse, happen to mine last summer. Daniel Defense BCG in a Bravo 11.5. I can't tell you what the cause was. Had some FTF and FTE until it quit. Didn't notice the damage until I broke it down. Does Daniel Defense have MIM gas keys? If not, is MIM the cause. Really I have nothing to add except that it's happened to me as well.
Looks like the gas tube was misaligned and coming into contact with the end of the gas key, to me. Did they replace your gas tube as well?
Looks like the gas tube was misaligned and coming into contact with the end of the gas key, to me. Did they replace your gas tube as well?
Nope, only the gas key. The tube is undamaged.
Nope, only the gas key. The tube is undamaged.
Did they replace the gas tube?
Did they replace the gas tube?
No, they did not replace the gas tube. The work order picture I posted is all they did to the assembly.
I'm thinking that they fired 60 rounds to function test it. If the tube on the gas key is not damaged now, but gets damaged again like it did the last time, then the previous post about a misaligned gas tube could be correct.
I'm thinking that they fired 60 rounds to function test it. If the tube on the gas key is not damaged now, but gets damaged again like it did the last time, then the previous post about a misaligned gas tube could be correct.
Could be. My tax stamp just came back, so I am actually going to swap the 16" barrel out for a 10.5" and sell off the 16", so I don't know if the process will repeat or not. Also going to swap the semi BCG for a full auto one since I plan to drop in a binary trigger.
I wonder if your missing, or have a busted bushing in the MRP chassis that indexes the straight gas tube back into the upper receiver portion.
Seen damage just like you reported from that.
I wonder if your missing, or have a busted bushing in the MRP chassis that indexes the straight gas tube back into the upper receiver portion.
Seen damage just like you reported from that.
I would be extraordinarily pissed off if that is the case. Considering that LMT had the upper in their possession for two weeks, if they didn’t figure that out and fix it, that would be a major black mark for their QC.
I would be extraordinarily pissed off if that is the case. Considering that LMT had the upper in their position for two weeks, if they didn’t figure that out and fix it, that would be a major black mark for their QC.
Just see if the gas tube moves when you push on it.
I would be extraordinarily pissed off if that is the case. Considering that LMT had the upper in their possession for two weeks, if they didn’t figure that out and fix it, that would be a major black mark for their QC.
Yours would be the second case I've seen of a problematic gas tube index bushing in since 2009....and it looks like yours isn't a case anyway. Your MIM gas key is the first one I've ever seen damaged, and I use the term damaged specifically instead of failing. I'm not sure we have any evidence the MIM gas key actually failed when an otherwise non MIM gas key wouldn't have. Could be a structurally failure of the material, but it also could be damage caused by an unknown culprit at this time.
You can see if your bushing is GTG, by shining a light down the chassis without the barrel installed. It's right above the barrel extension hole obviously.
You can see if your bushing is GTG, by shining a light down the chassis without the barrel installed. It's right above the barrel extension hole obviously.
What is it supposed to look like? When I shine the light down the hole with the barrel removed, it looks as I think it should...but with nothing to compare it to, I could be wrong.
What is it supposed to look like? When I shine the light down the hole with the barrel removed, it looks as I think it should...but with nothing to compare it to, I could be wrong.
It's a triangle that has the tip pointing towards the top rail....
https://i.imgur.com/YK6MXYM.jpg
Are the keys on the Enhanced Bolt Carrier Groups also MIM? For what you pay they better not be! (I have 4 of them)
It's a triangle that has the tip pointing towards the top rail....
https://i.imgur.com/YK6MXYM.jpg
Thanks for that detailed pic! I will have to check this afternoon once I get home to see. I also have a newer MRP upper, so I will pull both barrels and do a side-by-side to see if anything looks different between them,
Are the keys on the Enhanced Bolt Carrier Groups also MIM? For what you pay they better not be! (I have 4 of them)
Pretty sure LMT developed the enhanced BCG after they stopped using MIM keys. I have 2 of the enhanced BCGs and neither of them has a MIM key.
You can see if your bushing is GTG, by shining a light down the chassis without the barrel installed. It's right above the barrel extension hole obviously.
Was able to take a look...removed both barrels from both uppers and shined a light in. I can see the triangle insert on both, nothing seems abnormal. Thanks for the tip though
Does anyone know which years of production LMT used MIM gas keys?
I have no useful info on when LMT did / did not use MIM anything.
In the past year I have purchased two new, non-enhanced, DI, 5.56 LMT BCGs from two different reputable suppliers. Neither key looks MIM. One key is phosphate / chrome-lined and the other key looks like Nitride - not chrome lined - on a phosphate / chrome-lined carrier.
Both BCGs look like straight-up ass.
In LMT's defense I have only made one attempt to contact them, which went un-answered. I have not pursued it further - really not interested in words coming from an organization that would ship such items.
Good on them for taking care of the issue.
Here’s to hoping the fixed the problem and not the symptom.
Does anyone know which years of production LMT used MIM gas keys?I do not have a concrete dates but I received a defective, malformed mim gas key on a BCG from LMT, which they quickly replaced with a new carrier with standard machined gas key. This was sometime around 2008-2009.
Sent from my BE2028 using Tapatalk
I have no useful info on when LMT did / did not use MIM anything.
In the past year I have purchased two new, non-enhanced, DI, 5.56 LMT BCGs from two different reputable suppliers. Neither key looks MIM. One key is phosphate / chrome-lined and the other key looks like Nitride - not chrome lined - on a phosphate / chrome-lined carrier.
Both BCGs look like straight-up ass.
In LMT's defense I have only made one attempt to contact them, which went un-answered. I have not pursued it further - really not interested in words coming from an organization that would ship such items.FWIW, I emailed LMT yesterday afternoon and asked if their standard L7D3 BCG's have a chrome lined gas key. They got back to me first thing in the morning and said: "The bore and gas key are chrome lined."
I've been saving up to get one in my build, so it was worth verifying, before I finally ordered one.
FWIW, I emailed LMT yesterday afternoon and asked if their standard L7D3 BCG's have a chrome lined gas key. They got back to me first thing in the morning and said: "The bore and gas key are chrome lined."
I've been saving up to get one in my build, so it was worth verifying, before I finally ordered one.
I wish you the best with your build. Hopefully you have a better experience with LMT components than I did, Cheers.
I wish you the best with your build. Hope you have a better experience with LMT components than I, Cheers.
I do as well and thanks. So, far everything I've bought from them in the past 6 months has been great quality.
I've definitely seen people report some bad QC in the past couple of years. I'm just hoping it's a matter of new workers, sending stuff out that they shouldn't have.
Just an update for anyone that comes across this again. Received my LMT L7D3 BCG and it looks excellent. Smooth phosphate coating, great machining/ staking and the gas key is indeed chrome lined (and obviously not MIM).
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