Upper receiver mounting hole warped?

[alphatanker3ID]

(I'm not sure if that is the proper name for it, but I'm talking about the rear of the two holes that the upper receiver is pinned to the lower with. See the attached picture for reference. )

The rear pin on my M4 Upper sticks severely when trying to assemble the rifle. When I say severely, I have to take a hammer and tap it in lightly (carefully obviously). I also have to use a punch and hammer to get it back out to disassemble. When just trying to punch it through without the lower, it goes just fine... so its clearly a problem with the upper.

I was told when I bought it that it would eventually "break in" but that has not been the case. I assume that the hole is either too small or warped. Does anyone have any experience with this? What would you recommend as a probable course of action? My first thought is to take a small cylindrical file and take off a little bit at a time from the hole... but who knows - I'm no expert. The only rifles I've used are service rifles that are so loose that you can pop that pin out by looking at it the wrong way.

By the way, I bought the rifle new (Charles Daly M4) from a reputable dealer. It has performed perfectly otherwise.

[bkb0000]

rear take-down lug, i call it. its common for these to be tight, and sometimes even really tight. i'd call CD and see if they want to help you out. otherwise, if you're stuck with it, i'd hit it with the file. you need to be able to get the weapon apart without tools.

[alphatanker3ID]

The rear take down lug... sounds good! I've heard CD's customer service is very good, but if it is a self-serviceable problem I'd like to do it in lieu of sending it off. Thanks for the input.

[bkb0000]

just be careful- most of the pole files/chainsaw files are pretty rough. try to find something in the "medium grit" area, like a serrated knife hone or something. i'd just polish off a thousandth at a time, it's not going to take much to get it perfect.

[Heavy Metal]

Sometimes, shooting is all you need to loosen them up. I use a rubber mallet and a delrin punch until they break in.