I recently purchased a used DPMS .22 complete M4 upper from another member.  Tonight I was trying to fit it to the lower that I wanted to use it on, only to find that it won't fit.  So, I drag all 4 of my ARs out of the safe, and it won't close on ANY of them with the bolt/carrier installed.  Without the bolt/carrier it will close on the Bushmaster lowers but not the Rock River Arms or the Cavalry Arms.

The way I'm assembling it is to line up the front pin, push it in, and then try to "hinge" the assembly closed.  It catches on the back of the upper/bolt/carrier when I try to do this.  I tried sliding the upper onto the lower thinking maybe I could compress the buffer/spring and get it in but when I do it that way the holes never line up.

Is there some trick I'm missing?

Not that I know of. Mine went on two different EA lowers with no problems at all.

Maybe now you know why it was for sale.

All kidding aside, if there's interference with four different lowers, I think it's safe to assume the problem is with the upper. Might be just a matter of a few file strokes to correct--I'd make them on the upper since it seems to be the problem. I had something similar with my EA lowers--safeties didn't want to go in until I'd finessed the right side of the hole a bit with a rattail file. It happens.

Try putting it on with the front pin out, put it in place and put the rear pin in and see how the front pin holes line up. That's where I'd start, anyway.

I've got a similar problem when I use my 22lr upper on my RRA lower.  The solution I came up with was:

1.  File the lugs on the upper to until the upper drops into the lower.  (I never really liked the filing option)  I had to file while tring to get a RRA upper  lower to close...remember never force anything, always use a bigger hammer.

2.  Slide the upper and lower together and then push the front pin in first.  The rear will be a little tight but it will go in.  I had to use the back of a screwdriver to "tap" the rear pin in.  It will be better over time (the effort required to remove & install the upper).

Hope this was a help

Quoted: 2.  Slide the upper and lower together and then push the front pin in first.  The rear will be a little tight but it will go in.  I had to use the back of a screwdriver to "tap" the rear pin in.  It will be better over time (the effort required to remove & install the upper).

I tried this and cannot get the front pin in.  If it would work this way I really wouldn't have a problem with doing it, but there is no way either pin is going in like this.

This thing doesn't require a buffer/spring right?  It's much easier to deal with now that I took those parts out.  I'm somewhat bummed because I wanted this to be an easy drop-in on my other lowers and not have to have a dedicated lower to make this work, but at least I can get it to the range & function test it like this for now.

The idea was to eventually have a dedicated .22LR suppressed upper that I could just pop two pins and drop on.  Not sure if that's in the cards now or not.

If you have the bolt assembly out of the upper, can you get the upper on and both pins in?  If not, then I'd figure out which way to file on the lugs and/or holes to make it fit.  Like Snake45 I'd say that if it's the same with all 4 lowers then it's probably the upper that's off.  

When the bolt assembly is in the upper does it fit flush with the back of the receiver?  If it is, and it's just hanging up on the top of buffer then just compress the buffer a little bit when installing.  If it's not, then I'd start looking at the front of the receiver where the assembly mates to see why it isn't going all the way in.

The bolt/carrier actually appears to stick out a little bit past the end of the upper receiver, and I think this causes the problem ont the lowers where the upper fits without the bolt/carrier installed.

Maybe Snake45 can tell you whether that's normal or not, but I'm assuming it's not.  Maybe there's a burr or a bit of crud that's keeping it from going all the way in?

Mine was also tight. I used a rubber/plastic hammer to drive the pins in. Once it "broke in" some they became normal fitting. If you get the front pin in, hinge it back and forth a bunch. Another option is a drill bit w/cleaning brush in the chuck. Get the paint/anodizing out of the uppers pin holes.

Quoted: The bolt/carrier actually appears to stick out a little bit past the end of the upper receiver, and I think this causes the problem ont the lowers where the upper fits without the bolt/carrier installed.

The bolt should be flush with the end of the reciever and not be interfering with anything. The pins on mine are tight but it hinges open and shut just fine.

Make sure the bolt unit is going all the way into the receiver. Sometimes the extractors can fail to line up with their slots and keep the bolt out of battery.

It's also possible the whole upper might be due for a little vacation "home." DPMS is pretty good about fixing stuff, I hear.