Funny I should run across this just now.

Last week, instructing an AR Armorer class, we were covering barrel change. I have an array of tools and demo several different wrenches for the barrel nut and tools for immobilizing the receiver. Some are not everything I think they should be and don't do everything I think they should do, but honestly, the worst among them seems to get it done.

I was showing how the barrel itself has no role here-- we are working with the upper receiver, the barrel nut, and the barrel nut extension. To demonstrate, I tightened a barrel up and then screwed the barrel out of the barrel extension (which I had prepped to be only hand tight). The barrel being there or not makes no dif, it's not in play. I then took a barrel extension only, one with no locating pin, and using the Geissele Reaction Rod to immobilize the barrel extension, started tightening. Absent the locating pin, the upper wanted to spin with it-- illustrating my idea that when the pin is in place it prevents this but a lot of torque may be imparted to it-- that tiny little pin bearing on a tiny slot cut into aluminum. Is that a real problem....? Apparently not! But it has always bugged me.

So-- I was lamenting to the students that one tool immobilizes the barrel extension, another immobilizes the receiver. And that the MagPul Bev block immobilizes both, in theory at least, so that only the nut spins, but I don't feel like it does it to the Nth degree. One student said "Hang on a sec."

He produced the MI-URR and DAMMIT why didn't I think of that!! To me it's the cat's ass for this job, holding the barrel extension and receiver in precisely the right relationship to one another, preventing side loading of the locating pin, guaranteeing 1/4 MOA groups with Chinese ammo. OK, slight exaggeration there but I ordered one that night when I got back to the hotel and had it a few biz-days later. Good stuff...... with the AR market being so flooded with mundanery, I absolutely love it when smart people come up with a great idea and then make it to a high standard. Definite stamp of approval on this.