MCX will be in my hands sometime this week. Can anyone tell me if an MCX chamber (300 BLK) is essentially the same as an AR chamber, such that I can use my Reaction Rod on it? Want to swap muzzle devices immediately and don't prefer to rely on a mag well vise block. Guessing the MCX upper also wouldn't fit into an AR upper vise block.

My Geissele reaction rod won’t work with my MCX chambered for 300 Blackout because the lugs on the rod don’t fit into the slots of the barrel extension and therefore the rod cannot be inserted into the extension for full engagement. Although I didn’t measure them, in looking at the MCX bolt and the reaction rod, the lugs on the MCX bolt are visibly narrower than the lugs of the reaction rod.

Thanks for the info. Have you tried an AR upper vise block?

I don’t know if there are different models/versions of the MCX, but mine is the “Virtus” and its upper receiver is shaped significantly different than the usual AR configuration. I didn’t want to remove the sight from my MCX to check the actual mismatch between my Brownells upper receiver vise block, but it’s obviously quite a bit and the block wouldn’t be usable, IMO, with the MCX.

A bit of research revealed that others have had the same questions. Midwest Industries specifically states that their receiver rod don’t work with the MCX Virtus, and I wasn’t able to find any affirmative answers to the vise block question.

After some experimentation, my Wheeler upper receiver block (attaches via the pivot pin holes) works on the MCX.

Just as an FYI to anyone else in this situation.

That's good to know. Thanks.

Me personally I would not use the pivot pin holes for barrel work. Lacking other direct to barrel options like a reaction rod I would use a set of barrel vise jaws of some kind. The ones I have work well for this (I used them to change the FH on my MCX and numerous other guns where I can't use the Geissle).