I put a shorty(11.5') upper on my DPMS lower and took it to the range. With new factory ammo I test fired it, after every two rounds the bolt would eject the spent casing, and then stop  just before it would strip a third round out of the magazine and  push it into the chamber.  Does anyone have a  reason/solution. I like this upper , but if it can't be fixed then I'm not buying it. I'm using a DPMS  fixed stock lower.Any help would be appreciated!

Alright,

I do remember hearing about problems with barrels this short.

Let me see, three things - some is advice and some is just FYI.

1) Advice: Check the gas tube/gas system. From what I remember the cycle rate is *VERY* fast when you have a short gas tube. IF IT DOESN'T HAVE IT ALREADY, you may need some type of adjustable gas system. Assuming the magazines are good, I believe the bolt it going into battery before the magazine has a chance to feed the round correctly.

2)INFO: I remember an article about the Army trying 11.5" barrels many years ago and having similar problems due the gas system. They eventually gave up.

3)Question: Why a 11.5" barrel??
I'm assuming you know the legality of it and have the appropriate paperwork.

A friend of mine had to drill the gas port of his 10.5" to make it larger.  Works like a dream now.  Seek a proffesional before you do this!

Truthfully...it just looks so fucking cool! I have two 16' ar15's and one 20' including a colt9mm. A friend owns it and wants to get rid of it for another TOY, I liked it and he told me he never has had the problem I listed on my original post.

is the casing getting stuck in the receiver and jamming the next round? if so, its your extractor...not enough tension.  get a d-fender insert or put a small o ring around the extractor spring.  a buddy of mine fixed his 7.5 upper like that.  apparently the shorter gas tube increases the perssure to the point where the bolt cycles to fast for the extractor to hold on to the casing.

Ok.  First short barrels are trouble.  I have an original EA Arms with an 11.5" plus 5.5 NFA extention.  It works without trouble for some reason(a smaller gas port?).  check out www.M16pigtail.com. there's another one but cannot think of it now.  The problem is excessive port pressure and bolt velocity/chamber pressure.  Its a tough dance act.  But it can be made to work.

The case is not getting stuck, I have complete extraction and ejection. The bolt is  just at the lip of the magazine-where it would strip another round off...but it stops. Fire two rounds and this happens,every time.

Although I don't have a short barrel, I had the same problem for a while.  I fixed it by refurbishing my magazines, installing green followers, and stretching out the springs which had taken a set to them.   Problem solved.
The rifle was clean, too.

Uh, you DO have that lower receiver properly registered with the BATF as a Short Barrelled Rifle, I presume...?

It wouldn't do at all to get caught with an unregistered NFA weapon.

CJ

try other mags?