I've got two barrels that both fit in my standard bushmaster A2 upper. They just slide right in. But my Vltor MUR just arrived and neither barrel fits in it. One I have stuck in it right now and its only half way on. Did I get an out of spec upper?

(update) Its fairly cold outside, 18F, so I put the barrel outside. Then I'll bring it into the garage where I'll hit the upper with a heat gun and hopefully it comes off

If a barrel is stuck and you want to remove it, a section of 1" wood dowel works well, just enough to go through the receiver plus a couple inches, then tap it and the barrel out with a mallet.

lube?

I recently had the same problem (Vltor MUR and Noveske 16.1 Recon barrel). I tapped it lightly around the edges of the barrel extension rim with a brass hammer to get the barrel to go in. I can't imagine this would do any damage, I wasn't hitting any harder than I was hitting to get pins into the lower, but I'm new to the whole thing. Maybe someone can comment.

I did spray CLP on the surfaces and it didn't seem to help. Not sure what's going on as my two barrels slide right into my other receiver. The Vltor I just got the barrel goes on 1/4 of the way and then really tightens up. I had to pound it on the carpeted floor (professional eh?) to even get it on half way. Anyone know the tolerances of the inner diameter?
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Update: Got the barrel off. It was 18F outside. I put the barrel out in the cold then took it into the shop and heated the upper with a heat gun. Slid right off.

PK seems to think its tight tolerances + parkerizing.

I get 0.9945" with a bore gauge. PK firearms recommended I take some emory cloth and sand down the parkerizing a little.

I assume the nominal is 1.000. Should I attempt to take 0.006 off with a little sanding?

Try hearing up the receiver in the oven to about 250-300 degrees. The aluminum will expand some and the barrel should slide in proving the hole is not way undersized.