www.predatormastersforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=5;t=004405#000015

Anyone got any ideas?

Thanks.

Rick

Why start another thread here? You all gots your list of things to check, but appears the fella and his 'smith do not know how to inspect the rifle..

Key torqued proper, clean flat surface, staked proper?

What ammo are we using?

All gas holes clear and aligned?

Place 1 round in a mag and fire,,, does it lock open on the empty mag? Need to know.....

What length barrel are we using? I'm not a member there, so I cannot post.

Tell him to send me the upper and $20, and a prepaid return ship label in the box.

I know how to break down an AR, and build it from scratch yet I cannot inspect one?

Give me a break! I have checked every single thing on that gun with no results. Tried the one round mag trick and no, it does not stay open. It is not traveling far enough back for the head to stop on the catch.

Tried 2 different gas blocks, including the pinned on A2 tower.

I like how people think I am a moron and that my key is loose, or I forgot to put on the gas rings, or I tried to install the gas tube backwards. So far this gun has seen:

-two different receivers
-two different buffers and springs
-two different hammers
-four different mags.
-two different carriers/bolts
-two different gas blocks
-two different disconnectors
-three different brands of ammo.

Only thing not swapped is the stripped lower and the barrel.

Have you tried using your upper on someone else's lower, and that person's upper on your lower? The problem should follow the faulty assembly.

From the sound of it, my guess would be gas port. I mean, it's just about the only thing left anyway that hasn't been changed. The port is at exactly 12:00-ish, right? I assume you checked for that too.

I bet you've already done it more than once, but you could try sticking a drill bit into the gas port and turning it by hand just in case it might have a big fat metal burr stuck in there or something similar.

Replace the butt stock screw, the one you have is to long.  They make two small ones. One is longer then the other. This explains why the carrier will not go back all the way and hold open.

After what you said from the above I would put some serious money on it. I had the same problem last year, my screw was a little bit to long and I had to grind it down a 1/4" and the gun never had another problem.

Best of luck.

Sooooo,,, can you lock the bolt open using the charge handle to pull the bolt-carrier assembly rearward and lock open on an empty mag?  If so, the "long screw" theory is a non-issue.

You still have not come clean and said anything about the manufacture of the barrel, the barrel length,, unless I have missed something somewhere. I have a 7.5" and 11.5" and 12" that all operate with no problem.


Who made the barrel?
What length barrel?
What is the current gas port diameter?
Is the gas port positioned to sync with the port in the sight tower?

I can manually lock the carrier back, and feed a whole mag of rounds through it manually with no problems. We checked the gas port and it looked fine too.

The lower is DPMS forged.
Upper is DMPS hi-rider
Barrel is DPMS Hbar 20"
Carrier is DPMS
Stock set is DPMS A2.

I had a RRA A4 upper receiver on it, but with the same barrel. It has got to be the barrel at this point, I have swapped everything else out except for the stripped lower and barrel.

Maybe I have missed, or not been brought up as part of the diagnosis, but I will assume the bolt will not lock open on an empty mag when a single round is installed to a mag, then fired.

Go back and remove the sight base/gas block and measure the gas port diameter. Typical 20" barrel will use a .063-.070" +/- diameter gas port.

Go to a machine tool supply store and get a couple of "number/letter" sized bits to work with.

1/16-- .0625
#52--- .0635
#51--- .0670
#50--- .0700
#49--- .0730

I have an old article  by John Norrell, he states typical 16" Colt barrel has a .063", 20" barrel would be similar in most respect.

An old NRA article with diagram/measures from USMC Match barrel from blank, lists primary bore with a #45 (.082) then finish with #43 (.089)..... I use the #45 on my 11.5"....

I feel the #52 through #50 should take care of your problem, if the port is indeed under 1/16" diameter. But verify the measure/synch of the hole on the barrel in relation to the hole on the gas block-sight base,,, there has been some where the blocks' hole has partially covered the barrels' port.

Load 1 round to an empty mag, fire, and see if bolt locks back... do it a couple times, then open port with next size bit,, repeat the 1 round in empty mag again a couple times, until the bolt locks open on empty mag.

You are correct in that the bolt will not stay open after firing with an empty mag. I wish I would have checked the diameter of the port hole, but I sent my upper back to DPMS today. I assume they will check this hole?

Wouldn't me drilling it out leave a jagged edge inside the barrel?

DPMS should have your barrel repaired pretty quick... I had to send back my brand spankin new DPMS Kitty-Kat 7.5" upper when it would not cycle. They turned it around to me in a week.

The QC tag on the barrel when I got it did not have the "test fire" with "X" rounds box marked. The repair ticket was marked "open gas port" and test fire with 3 types of ammo. Works fine for me for 4 yr now.

If you use a new drill bit, and go slow, should have no problem. I did my 11.5" back in '90 when the barrel was new, would not cycle fully, so I drilled the port to same as the 11.5" OLY barrel I had that worked fine. Even if you have a slight burr (as seen by few inexpensive kit users), a couple bullets down the pipe will remedy that.

I think your repair will be a simple thing.
Good luck with it.

The J&T kits come with a nasty looking bur on the inside of the barrel from the gas hole... they disappear after a few shots.

It kinda freaked me out the first time I saw the bur, but people said it was normal and to just shoot it out, they were right.

Thanks, I will post what they found when I get it back. The only thing that is good about my AR not working is it kept me from building another one! They are addicting just building one!