Seeking assistance with Sig Sauer M400 malfunction

[Khaot1c]

I experienced a malfunction with my Sig Sauer M400. Other than some Magpul furniture and a Burris optic, the upper and lower (internals) are all stock.

Once I was able to get the bolt to release (it would not lock and it took a little (careful) prying to get it to ease out of the chamber. Once I could see into the breach, I discovered that a small "nut" (for lack of a better word) inside the breach cavity had worked its way loose. As the picture shows, its in the space between the bolt lugs and the bore.

The 'nut' was inside the breach and the bolt lugs had wedged up against it which caused the action to sieze up. There also seems to be a small gouge in the upper where the charging handle rides. My guess is it tried to migrate out of the action but was caught by the bolt before it could escape out the ejection port. Please see the attached picture. my apologies for the poor quality. auto focus wouldn't' cooperate with me and let me 'focus' on the hole - it kept picking up the edges of the upper...

I haven't seen this "nut" before but I only have 2 other AR uppers to compare with. The nut is VERY small and doesn't even have any discernible way to install it. IE: No cross-tip for a Phillips screwdriver or hex-head for an allen-stock.

I plan on calling Sig Sauer to discuss this, but I would like to have some background info on "what" this is.

Any info you guys can give me would be great!

thank you!

[chasetopher]

Take a quick look at the bolt carrier key's fasteners, on the potential of there only being one intact.

[b225]

Hidden inside, just aft of the chamber, is a protrusion that lines up with the extractor when the bolt is in battery. This button-like addition supports the extractor body when the gun is fired. Only 1⁄3 of an AR’s extractor is contained within the barrel extension when the bolt is fully in battery, leaving the remaining length retained only by the extractor pin that also acts as a hinge for normal function.

It looks like this part broke off or worked free.

Link

[Khaot1c]

It looks like this part broke off or worked free.

Link

Yes sir, I do believe this would be the culprit. I can't see in the hole very well to tell how much is left in it. There are about 4 threads on the small, and by small I mean TINY piece that came out. I don't see any sign of any lock-tite on what threads are left......

[BTL BRN]

Not trying to kick you when you're down, but this may be an example of a solution looking for a "problem." I was curious as to what Sig mean in their adverts by "supported extractor."

Thanks for the pics.

[chasetopher]

I learned something new today, I was under the assumption that the m400 was only different from a standard AR with the lower receiver 'additions'.

It will be interesting to see what they have to say about this.

[Khaot1c]

Heard somebody say something one that started with "If it ain't broke......"

but in all seriousness. The lack of Loctite has me concerned. I'm sure they have the "tool" to install it and apply some amount of torque to it. Given everything that's going on there, why wouldn't' they secure it better?


It's possible I'm the only guy this has happened to, but I have to doubt it..... Even though I think I have the worse luck of any human still living.

[Shao]

What if there was an overpressure problem from a bad cartridge and it saved you from completely destroying your rifle? What if it just accomplished its job?

[Khaot1c]

Good point....

I'll give Sig a call in the morning and let you guys know what they say....

Thank you to b225 for pointing me in the right direction. My Interwebs searches weren't uncovering very much.
Thanks again!

[aguila327]

I have no doubt that sig will fix the problem. I am curious though to see how long it takes. Can you keep us informed. I've always had issues with the time it took them to get my pistols bCk from them. Just chalked it up to less manpower.