Questions on Sig M400 Barrel Coatings

I purchased an 11.5" barreled upper from Sig and I'm having trouble getting much information about the barrel. The interior of my barrel and chamber is silver but I always thought a nitrided barrel would be nitride colored (black) on the inside as well. Is this incorrect? Is it possible my barrel is bare on the inside or chrome lined?

The only markings I have found on the outside of the barrel are 5.56 NATO 1/7.

I called Sig and asked about the barrel but the only useful thing the person told me was they batch test their barrels and that they were nitride coated. I asked if they chrome lined them and they said no. But there is no consistent information anywhere about the Sig M400 barrels. I guess they started with cold hammer forged chrome lined barrels but have gone away from that. I have seen statements on the ever authoritative internet that they use 4150 CMV, 4140, and 416 stainless.

I'm going to pull the flash hider off tonight to see of I can tell anything from the muzzle end. I'll also try to get the ALG handguard off but the screws are really tight. I'll put the upper on an action block tonight and try but I don't want to booger it up.

I mostly just want to know what I got. For the price I'm still going to shoot it until it wears out. Thanks for any information.

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Originally Posted by JB2000

I purchased an 11.5" barreled upper from Sig and I'm having trouble getting much information about the barrel. The interior of my barrel and chamber is silver but I always thought a nitrided barrel would be nitride colored (black) on the inside as well. Is this incorrect? Is it possible my barrel is bare on the inside or chrome lined?

The only markings I have found on the outside of the barrel are 5.56 NATO 1/7.

I called Sig and asked about the barrel but the only useful thing the person told me was they batch test their barrels and that they were nitride coated. I asked if they chrome lined them and they said no. But there is no consistent information anywhere about the Sig M400 barrels. I guess they started with cold hammer forged chrome lined barrels but have gone away from that. I have seen statements on the ever authoritative internet that they use 4150 CMV, 4140, and 416 stainless.

I'm going to pull the flash hider off tonight to see of I can tell anything from the muzzle end. I'll also try to get the ALG handguard off but the screws are really tight. I'll put the upper on an action block tonight and try but I don't want to booger it up.

I mostly just want to know what I got. For the price I'm still going to shoot it until it wears out. Thanks for any information.

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Read this article. Sounds like you have nothing to worry about. https://www.shootingillustrated.com/...rrel-finishes/

Thanks GH41. I learned something new from that article. It sounds like the barrel is probably nitrided on the inside even though it isn’t black. My guess is it’s your average nitrided 4140 steel.

I don't think it is technically feasible to nitride the exterior of a barrel without also nitriding the interior.

Larry,
That’s what I’ve gathered but I've never seen it explicitly stated.

I put an H2 buffer in it and put 200 rounds through it without issue. The last 20 were PMC Bronze and they cycled fine. I would say it’s over gassed.