I've finally got all the parts to assemble my Mk12 Mod 0 upper. I've got a 18" .223 Wylde SPR Stainless Steel Rifle Length AR 15 Barrel w/ Ops 12, Premium Series that I had an Black Ionbond coating professionally applied and a Aero Precision A3 Upper Receiver however the fit is very tight. The barrel extension won't fit into the upper receiver alone. I could use some Aeroshell 33 that I'm going to use for the barrel nut threads to try and get the barrel to slide into the receiver however I do not want to effect accuracy. My question is

1. Should I use grease to get the barrel into the upper receiver and if I do will it come back to haunt me latter by effecting accuracy or

2. Should I try to get the barrel in dry with a mallet. If I do with such a tight fit will it be next to impossible to get the barrel out if I ever need to replace the barrel.

I should mention that this is my FIRST build so I'm a little nervous and don't want to make any mistake with expensive parts. Your prompt replies will be appreciated.

No grease on the barrel extension.  

The correct way is to heat the receiver with a heat gun, freeze the barrel in the freezer...press fit together.  

Seat it fully quickly.

Comments in red

The heater and freezer idea posted above should work also.

By the way, I have that same barrel in my Mk12 and have been very happy with it

Edit: Also check for any burs that might be hanging things up.

This.

Every BCM receiver or quality barrel I've ever used was like this.  I usually heat the receiver and freeze the barrel.  Every now and then the barrel will fit about 3/4 of the way in. I'll lube the threads and used the barrel nut to bring the barrel the rest of the way in by hand, no tools, tightening a little at a time. Loosening to check and tighten again.

I have had a few upper/ Barrel combos that were like this and I would prefer all of them to be like that because it would help accuracy.  The heat and freeze thing is one way but an easier way is to get a green fiber Scotch Brite pad and rub it inside the upper receiver to smooth it out.  Then apply a very small amount of oil to the extension.  It will still be a tight fit but go together much easier.  Sometimes the coating on the inside of the upper receiver is a tad too thick and smoothing it out with the Green fiber Scotch pad helps.  Do not use sand paper or anything with steal in it that can remove material.

9 times out of 10 you don't need to freeze th barr l.  Heat the upper.  It's thin and aluminum and won't take much.

I had to do this with my BCM upper and DD barrel. I put the upper in the oven at like 200-ish degrees and put the barrel in the deep freeze. Take the barrel out first as steel hold heat longer than aluminum. Then grab the upper from the own with a oven mitt and jam the barrel (with grease on it) into the upper. Took me a few tried to get the timing down but on the third or fourth time it slipped right in with minimal effort.