Hey guys I need some advice here, I just put together a 20 inch A3  both upper and lower are DPMS, anyway I take it to the range today, and here we go   shoot one  ejects fine but it wont slam another one home leaving me a empty chamber, I try the same magazine in my Armalite M15A2   works like a dream.  so I take about the bolt clean it and lube it up install magazine with American Eagle 55gr fmj's  shoots one  and will not send another one home......ok  I am stumped

Could you give us a little more detail dpmmm? Does it lock open on the last round from an empty mag? Does the bolt lock back manually?

more details on this       when firing it will shoot one round  just fine, it will eject the spent casing, and close the bolt back to firing position, but not pushing another round from the magazine to chamber, it acts like it is chambering up, but comes up empty handed

So I gathered from your first post but that doesn't answer the obvious questions.

Does the rifle lock open on the last round out of the mag and can you lock the rifle open manually?

I havent fired a full magazine thru this one yet, I can manually open and lock the bolt. Like I said in the 1st thread, you insert a magazine, bring back the charging handle, a round goes into the chamber, fire the weapon, the bolt blows back and ejects the spent casing, when the bolt goes forward it doesn't grab a new shell to the chamber, it just closes,and is in cocked mode,  I tried this about 20 different times today, each time I was figuring  that it could be the magazine, so then I tried the same magazine in MY Armalite M15A2 20"   and the magazine worked just fine

Need the flight path of the ejected spend case and the distance that the case flew from the ejection port, such as to the 2:00 position and 3 feet from the port.  

Also, what ammo were you using?

I believe your gun is short stroking.  If you put an empty case in the chamber and a round in the mag you will notice the empty will get extracted before the bolt goes back far enough to get behind the next round in the mag.

It is also not getting far enough back to get behind the bolt catch.  Thus the questions about locking open on the last round.

A not uncommon problem is a loose carrier key.  The screws are staked so you have to take an allen wrench to them to make sure they are tight.  Screws have broken with the head being held by the staking making them appear fine.

With the bolt closed on an empty case in the chamber you can check for gas leaks by blowing in the muzzle.  There should be no perceptable leak.

Rough sticky chambers can slow up extraction and agravate short stroking but yours seems extreme for that.

If you have a carbine stock make sure you don't have the longer fixed stock spring.

Put just one round in the mag and see if the bolt will lock back and stay open.

Sounds like a short stroke problem to me.  A couple things to look at..  Check the bolts on the carrier key for tightness and also make sure there is a pin in the side of the sight tower where the gas tube goes in.