I'm assembling my first build, and I was about to install my barrel last night, and I ran into a problem. My barrel would not slide into the upper receiver. I used my aeroshell grease and I could not get it to slide in. I read on a thread that a few people recommended using a block of wood on the muzzle device to tap it in, so I tried that and it would not full go in. So I stopped before I really had to hit it hard. I will go buy a dowel tonight to pop the barrel back out. My issue is this: I have an Aero Precision Enhanced upper http://aeroprecisionusa.com/aspx/store/productdetail.aspx?pid=252 and since the upper has an extension for the hand guard, I can not see what my problem is. I know the feed ramps do not line up with the barrel and the ramps on the upper (they are off by roughly 1/8 to 3/16 of an inch). I cant see if the alignment pin is slightly off or too large. The barrel is made by Black Hole Weaponry (10.5 300 BLK) with the following Black shot peened finish http://www.blackholeweaponry.com/Finishes.php .

I have been reading today about using a lapping tool, and/or "hand finishing" with wet/dry sand paper. I have no back ground with this. I am fairly mechanically inclined, but I also have not done this before and I don't want to make a expensive mistake.

Any help or advice would be greatly appreciated. I am feeling kind of disheartened today and am hoping that my tapping (and to be honest, a few medium force hits before I came to my senses to stop)  did not do any damage and that I will be able to tap the barrel out with a dowel and start over, but I am unaware where to go from here. I imagine the sanding is the way to go and hand fitting, but I am open to all suggestions. I know my upper is complicating the issue, but I do like how the hand guard attaches and already have the upper and hand guard purchased, so I would like to make it work.

Thank you

Maybe a dumb question, but did you remove the barrel nut to expose the threaded part of the receiver?

 










 

When I put together a Mega Arms MA10 it consisted of a Mega Arms 308 Monolithic upper receiver and a Noveske 18" barrel. The barrel was VERY VERY tight in the upper receiver. I read all I could and most people were saying to freeze the barrel overnight (to contract the barrel) and heat the upper receiver (to expand the upper) prior to inserting the barrel. Everyone talked about holding a block of wood on the end of the barrel and hitting the wood with a hammer to push the barrel into the receiver. I did not like the thought of doing that at all.

This is what worked for me and it was very simple:
1. OPTIONAL - I had no problem with freezing the barrel. Freezing it doesn't hurt anything. OPTIONAL - You can heat the end of the receiver using a propane torch or a heat gun if you like. I have torched a lot of receivers and it does not harm the finish at all.
2. Slide the barrel into the upper receiver by hand as far as you can. Be sure to align the pin with the notch. You don't have to push real hard or anything.
3. Slide the barrel nut over the barrel. Engage the threads and begin tightening the barrel nut. The barrel nut made contact with the barrel extension and pushed the barrel right in.
4. Torque the barrel nut.

Now if you find that the threads of the barrel nut will not engage or you meet with what appears to be abnormal resistance ... there could be something wrong. Don't force it.

OP, I had a similar issue. I had a Seekins upper and a Rainier barrel. I tapped the barrel in, but still had about 1/4" or more to go. I did as ARgonot said and installed the barrel nut. Everything went together just fine and no functional issue whatsoever. I didn't heat or freeze anything though, but it makes sense.

 

If you look at the receiver in the link, the barrel nut for this set up has threads on the outside of the nut, and it threads inside of the extension built on the upper receiver.

I never heard of freezing the barrel, I can try that. When heating the upper, will that effect the finish?

Thank you! Keep the ideas coming. If it is a problem with the upper or barrel, I can always send them back. Like I said, with a little force, I'm only a little ways away from getting it in. Perhaps I'll know more when I get the barrel out and see if there are any marks indicating any problems with the alignment piece or not.

There's still a barrel nut that needs to come out before the barrel can go in.

You mentioned the feed ramps don't line up... are you sure the index pin is sliding into the slot?

No, applying heat with a small handheld propane torch will not harm the finish in any way. Using a torch is very common practice, especially when disassembling store bought ARs. Manufacturers like to use Loctite on just about everything. I've had to heat my handguards to get them off of the barrel nut. I've had to heat the barrel nut to get it off the receiver. I've had to heat receivers to break down Loctite in order to remove barrels. I've had to heat gas blocks to get them off too. You're not holding the torch in one spot for a long period of time until it gets red hot or anything. Just brush across the surface, rotate the part and you're just heating it up. It will not harm the finish at all.

You don't have to freeze the barrel or heat the receiver before inserting the barrel. Doing so 'might' help a bit, it wouldn't harm anything, but not required. I doubt any manufacturers do that. What a pain that would be.

The Mega Arms MKM upper I bought has an extremely tight fit. By hand I could always get it about 1/4" away from where it needed to be for the correct fit/alignment. I called Mega and asked about the issue.

Guy on the phone said just to heat up the receiver a little using a heatgun or blow dryer. He recommended not freezing the barrel for some reason. Turned the blow dryer on high heat, warmed up the receiver for 20-30 seconds, and the barrel slide right in. When I needed to remove the barrel, I applied heat with the blow dryer again and barrel came right out. Not damage to receiver or finish.

That's good to know. That kinda makes sense too. A frozen barrel in contact with a receiver you want to be hot does sound silly. The frozen barrel may act like a heatsink.

Is the barrel extension sliding into the upper receiver most of the way, or is it not sliding in at all? Have you taken a caliper, and measured the outside diameter of the barrel extension, and the inside diameter of the upper receiver?

On another note, if the barrel extension is sliding into the upper receiver; for the upper receiver portion of the M4 feed ramps to be off 1/8th to 3/16th of an inch with the barrel extension portion of the M4 feed ramps, it does sound like the index pin is not aligned with the notch in the upper receiver.

Thank you all for your advise and your responses! I tried a combination of heat on the upper and putting the barrel nut on and tightening it down, and it worked with no problems and slid the barrel into place. No damage anywhere and the barrel is in place. Only issue was that when I was going to tighten the barrel nut to 30 pounds of torque for "mating" that a tooth on the special barrel nut wrench snapped. Not sure if I didn't have it fully in place, if the wrench was bad, or if my torque wrench malfunctioned. I'll have to order another wrench from Aero, so I will have to wait a little while to mate the barrel nut to the upper and diagnose that problem (and check my torque wrench) but that is minor compared to what I was dealing with.

Once again, thank you all! I really appreciate the help and the knowledge that is shared with me on this site... I will no doubt look into becoming a paying member!