Hey all.
I’ve gotten a lot of help from these forums recently, thanks for that! Hopefully this will be the last time I’ll need it! (Probably won’t be though) :)
So I own a Sig M400 Tread and I’d like to have a 15 inch BCM MCMR handguard. I contacted SigSauer and they didn’t really know the answer so I contacted BCM and they said “You will need to first verify that the Sig upper receiver assembly is able to accept a Mil-Spec barrel nut. All BCM handguards utilize a proprietary barrel nut desinged to work with a Mil Spec upper receiver assembly. If the Sig upper can accept a Mil-Spec barrel nut then you will also need to verify the outside diameter of the current gas block installed. BCM handguards have limited space available under them for our BCM low profile gas blocks.” So I ask you guys, will it work? Or am I screwed?
Thanks!

You may want to see if you can get your barrel nut off before you purchase anything. I Googled “Sig Tread barrel nut removal“. Most threads didn’t have an answer and ended with frustration. The only one I saw that did have answers was on Sig’s own forums.

Based on the thread titled “Tread Barrel Nut”, it looks like you’ll likely need to purchase an armorers tool such as the Midwest Industries barrel nut wrench and grind away one of the three prongs to create a two pronged tool for Sig’s unique tooth pattern on the barrel nut. The poster mentions leaving the prongs at 90 & 270 degrees.

One post in that thread also mentions that a gunsmith found that Sig used Red Loctite (or equivalent) to hold the barrel nut on. The gunsmith had to use heat to remove the nut. Sample size of one there, so please don’t take a torch to your barrel on my limited research unless you’ve exhausted all other options first.

Edit: One potential silver lining is that of the people who succeeded, nobody mentioned having trouble using the barrel nut of their replacement handguard. Although their silence on the topic doesn’t necessarily guarantee compatibility.

Yea I’ve heard it’s a huge pain. I hate how Sig advertised how customizable the Tread is but make it incredibly difficult to do so. I think it will fit, waiting on a email that confirms how large the gas block is, if it does I’ll take it to my local gunsmith to fool with it. It’s my first AR so I was naive and just got what I could get ahold of, you live and you learn I suppose.
Thanks!

My MCMR handguard measures a fraction shy of 1-5/16” diameter inside the handguard. That does not account for the additional space at the top of the rail for the gas tube.

It looks like you only have to remove two screws to pull your handguard off. Take it apart and measure your gas block diameter across its widest point not worrying about the upper gas block protrusion for comparison.

I’d measure gas block clearance for you on my rifle as a reference, but it wouldn’t help. I’m running a less common .625 gas block that has more clearance than a conventional .750 gas block.

Edit: It looks like Samson SXT handguards are a direct fit to the Sig Tread. Their barrel nut and tool make me wonder if Samson didn’t act as ODM for Sig’s handguard system.

Topic Moved

I'd find out if it accepts a standard barrel nut. If it doesn't, I'd sell that POS ASAP. Nothing worse than being stuck with proprietary design AR.

I'd pull that handguard off, torch the barrel nut and use a PIPE WRENCH to remove the barrel nut. There's no way I'd buy a barrel nut wrench and modify it just to work with some proprietary piece of $hit barrel nut. Then I'd toss that nonsense in the trash and replace the nut & handguard.