Once I found what seemed to be the right ammo (CCI Mini Mag and Winchester Dynapoint), my DPMS dedicated .22LR upper was running great. It was accurate (for a .22LR semi auto), and it was reasonably reliable, failing to cycle maybe 1-3 out of 100 rounds.

Well, after a few months, the reliability slipped. I went a few trips to the range without a cleaning, and chalked my problems up to a dirty rifle. So, before my trip today, I gave the rifle a pretty good cleaning, and things were worse than ever. I had about a 95% failure to cycle. So, I cleaned the rifle at the range, again, and again, and again. After each cleaning, the rifle would cycle correctly for 10-15 rounds (if I was lucky), and then would quickly degrade back to a near 100% failure to cycle. The failures are nearly always the same--the round feeds correctly, and gets about half way into the chamber. At this point, I can pull the bolt back a small bit, and when it slides forward, it drives the round into the chamber, and can fire.

So far, I have cleaned the chamber and feed ramps with Break Free CLP and a nylon brush, as well as the bolt carrier, and the magazines. I have tried running the bolt carrier wet with CLP, as well as dry.

After working correctly for a few thousand rounds with the same ammo I tried today, I am not sure why the rifle is not working. Could apart be broken, or is dirt accumulating in some spot I have not thought of.

Any suggestions?

Update:

I gave the chamber a very thorough cleaning with Tetra Bore Scrub. After that, I cleaned again for Hoppes 9. Went to the range, and same thing. I then lubed the carrier with Tetra Gun Lube, and it worked so-so for about ten rounds, then back to misfeed every time.

I called DPMS, and they said send us the upper, and we will fix it, and get it back to you in about two weeks. I guess I will give this route a try, though this leaves me without a .22LR upper to shoot. I wonder if my wife will notice if I buy one at the gun show this weekend?

First, run a USGI M16 bore brush through the bore. That will clean out the chamber, which your nuylon brush might not be doing.

Next, check the ammo. Is this malfunctioning ammo from the SAME BOX as the stuff that was running okay before? It's not unusual for lot to lot variations in these cheap ammos. I've had two boxes of the Fed Bulks that were oversized and refused to chamber in some rifles. The next box will be just fine.

Is the inside of your upper clean? Does the bolt move freely back and forth?

Is it clean UNDER your extractor? Dirt under there can hinder the next round from feeding up properly, bringing things to a stop as you describe.

Does your hammer cock easily by hand? Could be some binding or friction there. Also see how smooth the bolt/hammer interface is. If the bolt is using up too much energy cocking the hammer, that could be your problem.

Do you have another mag to test? This could be a mag problem. Have you dropped it or done anything else that could have bent the lips even a little?

If you are only using a nylon brush and clp it is very doubtfull you are getting the lead out of the chamber. Brass brush and lead solvent. Most likely your chamber is full of wax and the front is leaded.

Quoted: If you are only using a nylon brush and clp it is very doubtfull you are getting the lead out of the chamber. Brass brush and lead solvent. Most likely your chamber is full of wax and the front is leaded.

That is a thought. I had been using Hoppes No. 9, but CLP was all I had in my range bag, so that was what I used today. I had been using a brass brush, and only used the nylon brush at the range today. I'll go back to my brass brush.

HOw accurate are these uppers.  The pfrase "accurate for a 22lr semi-auto bothers me, my 10/22's will punch one ragged hole at 50 yds and clost to that at 75 with no wind.  

If cleaning doesn't work, consider taking the bolt appart and checking the firing pin.

A friend of mine had a firing pin to break after (~4000 round I think) and it cause failure to fire problems (as well as some ejection and feeding problems).

Good luck.

Quoted: How accurate are these uppers.  The pfrase "accurate for a 22lr semi-auto bothers me, my 10/22's will punch one ragged hole at 50 yds and clost to that at 75 with no wind.

My best 5 shot group at 50 yards yesterday was about 2 inches, though I was shooting with stock iron sights, and having to drop my magazine and pull the charging handle after each shot. I was shooting off sand, though was getting very frustrated. At 100 yards, I had most shots in the black on an SR-1 target. I can shoot better when I am not frustrated, though.

Quoted: If cleaning doesn't work, consider taking the bolt appart and checking the firing pin.

That is something for me to check. In my case, the rounds would fire every time I got the cartridge in the chamber, so I think my firing pin is probably good.

I noticed the one of my extractors was not lining up completely with the slot cut in the aluminum sleeve and the cuts in the chamber. Next week when I have time, I'll take it apart and see if things can move or if the machining is just off a little. I can open the cuts up no problem, then the extractors won't click on chambering, retarding the closing. I can probably bevel the front of the extractors so they slide into place easier.

Today is the only time in 1000 rounds it has had problems, same deal when I rdropped the mag and pulled the bolt and let it fly it closed all the way.

I've been shooting mine with a gemtech 96d and cci subsonic, man is it quite!

I had exactly the same problem with my DPMS AP4 upper.  After about 4000 rounds it just stopped firing. Take the firing pin out and clean it as well as the chamber it sits in. Clean it well, blow it out with compressed air and lightly lubricate.

Thats all it took for my upper to start firing again and its ran perfect ever since.

John