Working on a more precision-oriented upper build and picked up a BCM blem stripped upper because I prefer the thermal fit. The particular one that came to me is so tight that my receiver lapping tool won't fit. I'm tempted to chill the tool and heat the upper so I can use it to square the receiver face, but worried it will get stuck in the receiver once inserted. Anyone deal with this before?

Yes, it’s happened to me a couple of times.  Solution is brain dead simple: don’t do it yourself.  For about $10 plus shipping Paladin Machine Shop Service, tp555(at)vfemail.net, will lathe turn the front for you and you’d have it back in a week.  Lathe turning is bound to be more accurate anyway.  Email him for current price.  He is located in Kershaw S.C.

Thanks for the tip

I've seen where some receivers were bored half way through from each end, and where they meet in the middle, they don't really line up very well.  It will leave a bit of a "step" in the middle that can cause a tight spot.

Tony

I just picked up a BCM blem and I’m afraid I might run into this problem. I don’t currently own the lapping tool. Have you thought about chucking the tool in a drill and then running it with some high grit sand paper to take down the OD of the tool?

The purpose of the lapping tool is to true the face of the receiver, so the dimensional proportions are fairly important.  While I'm the kind of guy who will do just what you suggest, it might not be within the capability of the masses.  I literally opened the throats on a Ruger 45 Colt revolver cylinder using emery cloth wrapped around a dowel, and that gun went from barely being able to stay on a paper plate @ 50 yards to shooting 2" groups from a RR.  The other issue would be now that lapping tool is only useful with the same uppers, FWIW.

I'd do it, IF I bought into the hype of the BCM marketing...  

Yeah I wondered about those issues too. I don’t know if you would have to take so much off that it wouldn’t work in other receivers.

As far as not buying the hype, I can see why the idea has merit. Some people shim the outside of the barre extension to stop movement, so it seems that a tight fit is thought to be important. I’m a little unsure of the value of the lapping tool. If I understand correctly, the point is so that the axis of the bore is as square to the receiver as possible. But once an optic is zeroed on a rifle, isn’t any deviation there accounted for? And is there enough variation without lapping for it to matter?

When you torque that barrel nut 30 - 80 ft lbs, it squeezes the nut around the barrel extension at the same time it pulls the barrel tight to the face of the receiver.  Starting out so tight you have to heat shit up is nothing but marketing hype, IMO.

A decent friction fit is all I need to have peace of mind.  If the barrel wobbles in the receiver tenon it is another story.