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;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 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.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.
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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.
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.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.
... Maybe Sig used some cheap subcontractor to supply them? ...
If your LGS is like most around where I live, the people at the gun store aren't either....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.