I just purchased a new SIG M400 16" barrel from CDNN for only $50 (unthreaded). It arrived today and the barrel extension has two holes drilled through the sides. One is larger than the other. One has a metal pin through it that appears to block entry of a bolt into the extension.

I've never seen anything like it in 35 years of AR experience. Us this common to SIG M400 series rifles, or has this barrel been demilled?

I don't know how to post pictures, but if you go to the CDNN website you can clearly see one hole in their photo.

After surfing the net I discovered this is normal for SIG M400 barrel extensions, now my question is, can standard AR-15 bolts work in this barrel?

If not, is there a source for SIG bolts?

Thanks.

For the record, this is considered an improvement from standard extensions.

If I’m not mistaken the HK416 has a similar setup
as well as the Sig 516.

Both my M400’s have it, the pin presses against the extractor when it’s in battery, it’s supposed to keep the claw from popping off the rim of the case as the bolt starts to unlock. Never had any extraction problems even with steel case ammo.