Built a DPMS rifle, the gun would not cycle properly, at first it would jam upon ejecting and would not eject the shell, we polished the chamber and that fixed it.

Now, the other problem is the gun was short cycling. Nothing I did would make it shoot. So I got mad and replaced the whole upper half including the bolt/carrier. I even changed the spring and buffer.

Took it out today and boom, it would jack out the shell but the carrier is traveling back only far enough to jack out the shell, but not far enough to get the bolt face behind the next shell to push it forward.

I tried a mag with only one round, hoping that it would shoot and stay open after hitting the bolt catch. It would travel back to where the carrier itself is stopping on the bolt catch, but it would need to slide back about another 1/2" to have the bolt retained on the catch. So it would either stop on the carrier itself or just not stay open.

I filed down the top of the catch thinking it was too high. That did nothing. Filed down the corner of the hammer thinking that when it is in the cocked position it was sticking up too high causing friction on the carrier. That did nothing.

What else can I do? I am thinking the only thing I have not replaced is the trigger and disconnector. For some reason maybe they are out of spec or something? It works when I charge it and keep my finger on the trigger and let off, it goes click. I can feed a whole magazine through it with the handle without a jam, it just won't fire right.

Could the spring be too long on the buffer?

I am at wits end. THANKS!

PS. I have tried 4 different brand mags including DPMS, Armalite, and Orlite.

You aren't using an A2 stock screw with an A1 length stock are you?

No, stock is brand new A2 assembly from DPMS.

Wondering if I need to cut the buffer spring down or something.

Hi Dogcatcher223! I have battled these short recoil problems also... good news, is that with new parts being the basis of the problem guns, all were made to cycle properly, bad news is, "There's new parts, and then there's good parts!" First of all, can you cycle the carrier all the way back with the charging handle? If you can make it catch on the hold open by doing this with an empty mag, then at least you know that if everything else is right, it will most likely work if we can just get it to travel back far enough... I have found, that when troubleshooting these types of these problems, that I could switch complete lowers, (tried 3 different lowers) of different makes, different parts, etc...  and it NEVER cured the problem...(Although, I oiled the hell out of the buffer tubes and buffers!)... I believe that the problem is in one of three areas....  1st, use of correct ammo to test the gun... XM193 Federal, Q3131A Win., Lake City M193 Ball, .... 2nd, Gas leak or restriction, in a new gun, the bolt carrier key would be about the only place it could leak, maybe the gas ring gaps lined up? I have never seen this, (the rings issue) personally make if fail, but I have been told so, and it most definitly would have an effect.... 3rd, and this is where I think the problem might be... some new uppers are pretty tight and I have found that the carrier assemblies drag on the interior of the receiver, most likely in the relief slot for the bolt carrier key... also some are tight in the middle of the round tunnel part of the upper... I have cured most of my problems in this area, by swapping out bolt carriers, and once it works, it works forever! Don't waste a shit pot full of ammo trying to wear it in. When the parts are correct, and the correct working tolerences established, it will work on the first round every time! To prove my point, if you take a dial caliper and measure the relief slot for the hammer on the bottom of the upper receiver, you will find that is is approx. .010" different from the front to the back... they were not machined this way... somewhere along the line they got squeezed a bit... The bolt carrier should run free and easy, and close all the way by just letting the recoil spring push it home... Remove the upper from the lower, run the bolt carrier in by just pushing on the back, if you feel it drag at all, that is where a potential problem exists....When all is correct, the bolt closes with a pretty smart snap!  all the above is my own opinions, and experience... but, I've got most all to run.. so it's the best advice i've got to give, good luck!....  till....later...GT

My old upper was a RRA A4, and the rifle did not function, and I swapped it with a DPMS Hi-rider. I also had a brand new FN carrier and swapped it for a brand new DPMS carrier. The carrier seems to slide very easily within the upper.

I have tried 3 different ammos so far. Black Hills, Ultramax, and Wolf. None of it worked.

I can easily slide the carrier back and forth with the charging handle and feed a whole mag of ammo that way.

The gas block I am using is a pinned on A2 tower that came with the DPMS barrel. I am stumped.

Hi Dogcatcher223! Damn, you do have a problem! Here's is what I would do at this point... First of all, I would see if I could get a working AR from a friend, and use his unit as a test base line, swap out the lower, try it.. swap out the bolt carrier ass'y, (check headspace!!) try it.... now your down to two parts...the upper... and the barrel...  I think the DPMS extruded type upper would be almost impervious to deformation, as it is quite robustly made... so, I'm betting ...either your barrel is the problem, or you still have a chamber problem...  brush the liv'en shit out of it with a chamber brush and oil...till it's immaculately clean, and try again...  I don't know about the other two ammo types, but for this type of trouble shooting, Wolf is a definite no-no! Wish you lived a little closer, we could tackle this problem on my home turf! till...later...GT

you might want to give Dave a call at DPMS, I had this problem with my 20" A3 upper,  it can be really mind boggeling.  I was using Federal XM193, Federal American Eagle in 55 gr and 62gr

Thanks for the input. I called DPMS and he thinks it is something wrong with the gas port or chamber. I am sending my whole upper to them tomorrow. Told me my lower is fine. Makes sense since the only part no replaced on this gun so far is the barrel! I am praying this is it, cause this gun is going to give me a nervous breakdown!

"I filed down the top of the catch thinking it was too high. That did nothing. Filed down the corner of the hammer thinking that when it is in
the cocked position it was sticking up too high causing friction on the carrier. That did nothing."

By all means, replace these parts.

I already ordered a new hammer and catch from Bushmaster. But, you could cut the whole back of the hammer off and it would not do anything. Only thing it needs is the flat face to strike the pin, and the hook to catch on the disconnector.

Hi Dogcatcher223,
Looks like you have tried everything. If you can cycle the action with out any binding, all the way to the bolt catch, mechanically it seems ok!
On one system a friend had a short cycling problem, it ended up being a gas problem. he had some  metal pieces blocking the gas block. So have you check the gas tube and gas block/or front sight assembly for blockage?? Or on another system, he had an"adjustable" gas block and had the adjusting screw dial way down!!!

I hope cure your problem fast
Stuart

Yeah, it has to be a gas problem at this point. I think the port is too small. Because with fairly minimal effort I can manually feed a whole clip through the weapon with no problem. Seems like it just needs a little more ooomph to kick the carrier back a little more. That leads me to believe there is not enough gas pressure. I will post what DPMS finds after inspecting my upper.

Check your gas key and screws.

Mine was loose and my rifle did exactly as you described