This is the 4th upper I have built and the 1st I have had an issue with.  The upper reciever, bolt, carrier and charge handle came from  Del-ton at the same time.   from the start I was having a short cycling problem,  so I went through the gas system,  reassembled the gas block and tube to check position etc, still had a problem with it short cycling.    I brought along  DPMS bolt carrier assembly from another rifle that I know works and sure enouigh it ran like a clock.

I know I had read a thread about this happening to someone else a while back but search turned up no results.    I see no marks on it except for brass marks on the corners where it rides over the cases in the mag.   Anyone have an idea what would casue the shrouded carrier to hang or have any other tip to look for when it cycles?   Appreciate any ideas, thanks.

Well waiting until the fingers thaw, a nights sleep and some coffee sure makes a difference in looking over stuff.

The upper has right at 70 rounds through, and the carrier has 50 on it, all in this upper.

Yes it passes the 45 degree drop test.  I see the carrier has wear on the shoulders on either side of the gas key, which is normal according all the other carriers I have.  But the upper has a pretty harsh wear spot on a corner ahead of the retaining pin notch.


My other uppers have wear there too but not quite as pronounced as this is.

The lower is a Superior Arms with a DPMS parts kit with a notched hammer if that makes a difference.   It has worked fine on 3 different uppers.  But this is the 2st shrouded carrier I have owned.

If I get a chance today  I will take another rifle out and try the carrier in it and see if it hangs in it also.

I am not sure about what the problem is. I had a MRP upper do the same thing. I sent it back to LMT and they replaced it. Probably doesn't help much.

Interesting.  I just put together a superlight upper with a shrouded bolt carrier, and it too is short stroking.   I've been through the gas system with a fine tooth comb and can't find any problems.   You can definitely feel a lot of drag as the shrouded carrier is drawn back across the (notched) hammer.  Much more so than with the non-shrouded variety.  I'll have to try a different carrier next time I get to the range.

Did you ever resolve your short stroke problem?  

Yep,  just yesterday I got out to run it and see.  It ended up being the gas port too small.

I put this upper all together anbd took it out to test to start with when it was pretty cold and it short stroked.  When I looked inside I saw the shiny wear mark and with the shrouded carrier being stiffer in action I had figured it was tha carrier dragging on something.  

However, when I went back out to test with another rifle and swapped everything around including lowers it was the upper that consistently short stroked, no matter what carrier I had in it.

So I went through the gas system, the PRI gas block had quite a bit of slop when the gas tube was inserted, yet was solid when pinned, but it had a little blow by on the tube and I suspected maybe that was enough to make it short cycle.

So I asked if that was normal for PRI gas blocks on the "Rifles, uppers ect" forum and got some ideas on how to fix that, when I mentioned the port size on the 18"  barrel, Bigbore suggested it was too small.  Sure enough I opened it up .010 to about .108 and it runs like a clock now.

If you can try another carrier, it may cure it, but I tried one and while it ran better it was still short cycling, it would not hold the bolt open on the last round.  Once you eliminate the carrier dragging, and have the gas system sealed up,  it really only leaves that the system isn't getting enough gas to work.


(Editted because brain doesn't work before soaked with coffee apparently)

Yeah, it never really occured to me that there may be too much drag from the carrier riding over the hammer.  But I'm just about of ideas, so I'm willing to consider anything.  What's odd about it, is the that the rifle ran fine for the first 6 to 8 rounds.  I fired one round at a time (fed from a magazine), and each time the bolt locked back as it should.  It didn't start short stroking until I had put 10 rounds or so through the rifle.  My first thought was that the carrier key had worked loose, but it hadn't.  I'm the one who installed it ....lapped it to the carrier, staked it, and put an index mark on the screw heads so I could see if the screws have moved.  They haven't.  I can't find a gas leak anywhere else either.   I guess my next step is to try a carrier that I know works properly.

Glad you got yours up and running.

Quoted: dragging inside the upper or dragging on the hammer? does it pass the 45 deg drop test?

What is this test of which you speak?

What is this test of which you speak?

With the upper removed from the lower, the carrier should slide freely within the upper, when it is tilted to a 45 degree angle.  If it doesn't, then something is dragging.


Oh, btw...I got my superlite up and running 4.0.   As I was going through the upper trying to figure out what the problem was, I noticed that one of the gas rings (which were new...as is everything else) had a rough spot on it.  It also looked like it was just a little (very little) bit sprung open compared to normal.  It didn't look like much, but on the outside chance that the rings were the problem, I replaced them.  That's the only thing I changed.  And now, it runs great.     No shroud dragging problems for me.

How about with the upper attached to the lower, say with no buffer or spring attached. Will there be slight drag of the carrier on the cocked hammer? Should it still pass the 45 degree test?

Hey Tweak, should the bolt actually rotate and lock when doing the 45 degree test?  Because  the bolt won't actually lock on either of my uppers.  Too much drag from the gas rings.  The carrier moves smooth as silk, though.