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Discussion Starter · #1 ·
Hey all,

I just bought an M400 about 3 weeks ago. I took it home, stripped it, and cleaned off the factory lube and replaced it with my own CLP. I took it to the range today and fired about 250 rounds, no mag dumps, or anything strenuous. The gun ran great all throughout. When I took it apart later to clean it, I noticed a large gap in the gas rings on the bolt assembly, and realized it was indeed blown apart slightly. I didn't find any splintering or anything crazy, and the barrel is fine.
My question is; is it common that these gas rings on a brand new AR to fail so quickly? Does this stem from a deeper issue? I called my gun shop and asked them if this is common, and they said it is, but I am military, and these rifles that we shoot are abused and have had many many rounds through them and I've never seen these gas rings break before.
 

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There is supposed to be a small gap in each one. And they are supppsed to be offset so all three gaps aren't lined up. One of yours looks like it's missing a piece? Maybe Sig used some cheap subcontractor to supply them? Good thing is they are cheap and easy to replace. The rest of the bolt looks fine.
 

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It definitely looks like one of the rings is broken. Are there wear marks on the bolt, and is the costing uneven on the bolt? The tail and mid area near the ejector looks shiny.
 

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It definitely looks like one of the rings is broken. Are there wear marks on the bolt, and is the costing uneven on the bolt? The tail and mid area near the ejector looks shiny.
That uneven wear would be my concern as well. Possibly the missing chunk of ring was jammed in there for a period causing the wear but it may be a machining tolerance issue. For replacement rings I would recomend the McFarland style;
JP Enterprises Enhanced One-Piece Gas Ring - .223 JPEGR-223
 

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Obvious, and as has been mentioned, you have a choice. First we need to find out why!
As mentioned it could have been defective parts, or the could be a problem in the bolt carriers bore, that caused possibly 2 rings to break...

If possible take a picture from the front of the carrier, into the bore, with the bolt removed, and a small flashlight shining through the rear of the carrier into the bore.
Post it, unless you see a physical flaw. If there was a machining problem, you need to let Sig fix it. If it is simply a gas ring problem, you should let Sig know. Replacing the rings with aftermarket parts, may fix the problem, but could also void the warranty if something else occurs.

It's up to you, also here are what the original gas rings look like, and the 1 piece replacement ring, as mentioned.
 

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I agree with Willard. I would suspect a problem inside the carrier bore that caused that. Gas rings should last for thousands of rounds.
 

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Discussion Starter · #9 ·
Hey everyone. Thank you for the replies! I am going to take the bolt assembly to the gun shop tomorrow and possibly call Sig and see what's going on with it. Here are some close up pics of it. Let me know what you think
 

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Thanks for adding the pics, however, I cannot tell anything from the view inside the bolt carrier, but I agree with Willard and animalhd1, most likely you have some issue with the bolt carrier being out of spec internally. It could be failing rings, but as they said, these normally last a long time before needing replacement.

Here's a shot of mine from my 516.



The pic is kind of dark, but there is very little wear on it, and the tail has the coating just like the rest of the bolt. The shiny appearance of the tail is very odd, but at the same time, I don't really see how that would have an impact on the rings. Hopefully Sig can send you a new BCG for the gun, without requiring you send the gun in.
 

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Thanks for adding the pics, however, I cannot tell anything from the view inside the bolt carrier, but I agree with Willard and animalhd1, most likely you have some issue with the bolt carrier being out of spec internally. It could be failing rings, but as they said, these normally last a long time before needing replacement.

Here's a shot of mine from my 516.



The pic is kind of dark, but there is very little wear on it, and the tail has the coating just like the rest of the bolt. The shiny appearance of the tail is very odd, but at the same time, I don't really see how that would have an impact on the rings. Hopefully Sig can send you a new BCG for the gun, without requiring you send the gun in.
That shiny appearance on his bolt tail is probably from cleaning the carbon off. The 516 will never get carbon in that area because it's piston driven. The gas rings actually do nothing on a 516 and aren't even needed. In this case under warranty I would call Sig and get them to email you a shipping label and RMR to send the entire bolt carrier group back for them to inspect and replace if necessary.
 

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That shiny appearance on his bolt tail is probably from cleaning the carbon off. The 516 will never get carbon in that area because it's piston driven. The gas rings actually do nothing on a 516 and aren't even needed. In this case under warranty I would call Sig and get them to email you a shipping label and RMR to send the entire bolt carrier group back for them to inspect and replace if necessary.


Good point on the appearance on the tail, and on the rings being unneeded on the 516. I've read where one forum member who is a gunsmith (can't remember his nic), recommends removing them on the 516 to reduce unnecessary friction, although I decided not to do try it on mine.
 

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Guys OP stated this is a M400, and not a 516, it's gas impingement, not piston, that's why there is carbon residue. That is carbon on the "tail" of the bolt. Theiceguy your pictures all appear to be of the bolt, and not of the inside of the carrier. That is what we need to see, as that is where the bolt and "rings" rides.
 

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In the rare off chance that the gaps on all three rings were lined up to create a single straight one instead of three offset it could have a created this. Almost every manual from every maker at some point has a 'warning' in it to make sure the gaps are offset. This is probably the reason for it.
Instead of the excess gas snaking its way thru three gaps offset it may have gone straight thru all three and caused that ring to break. Other than that I'll go with my first post, cheap or faulty ring and it failed. Very uncommon problem. I'd replace the rings and carry on.
 

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Discussion Starter · #15 ·
Hey all.

I cleaned the bolt carrier group pretty well and didn't find anything out of the ordinary. I am going to take it tomorrow to the gun shop and see if they have any input on it. I'll keep you posted on it and update once I get some kind of word.
 

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Guys OP stated this is a M400, and not a 516, it's gas impingement, not piston, that's why there is carbon residue. That is carbon on the "tail" of the bolt.
Yes, I knew it was an M400, but the bolt is the same on both models, and I believe the internals of the carrier are the same as well. I did not realize that was carbon on the tail, as it doesn't look like the carbon build up I see at the piston on the 516.
 

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The wear on both sides of the extractor in the first pictures is what caught my eye.
 
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...I still am going to take it to the gun store to see if there is anything out of the ordinary since I am by no means a professional when it comes to this.
If your LGS is like most around where I live, the people at the gun store aren't either. :D

I'm glad they are sending new rings for you. I love my M400.
 
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