I have 3 different uppers that do the same thing on 3 different lowers.
After firing, the carrier only goes back about half way w/ the empty case still on the bolt face.
The bolt/carriers all work in MY favorite AR. So they have been eliminated as the problem.
The free float tubes nuts are all torque’d to approx. 80 ft/lbs, the gas tubes are aligned in the upper, carbon under the gas manifold on the barrel appears to be concentric w/ the gas port in the barrel, the gas tubes seem to be clean.
Every thing appears straight and properly aligned.
I assembled these my self. Where did I go wrong?

Are the gas tubes installed correctly with the "low end" go into the gas block? Are the roll pins present?  Check the OD of the end of the gas tube that fits into the carrier key. Compare it to the ID of the carrier key, should be just enough difference to allow a slip fit. Are you using slip on gas manifolds? Pull them off and make sure the gas port in them goes all the way into the gas port. Use a twist drill and a bright light to check.

While I doubt it's possible it could happen with three uppers assembled in sequence check the ID of the gas tubes too.

How long are the barrels and how long are the gas tubes.

Did you headspace these rifles?

They are all 20" barrels, one colt mil. profile, one Bushy NM, one DPMS SS Bull. All are slip one set screw manifols, the DPMS is theirs, the Bushy is a their BAMS 3 rail w/ the flip up sight and the colt has a blue aluminum thing one I got cuz it looked different. The uppers and gas tubes came from different suppliers but were all purchaced at the same time. And as far as I know, there is only one inlet in the gas tube, so I you put it in upside down, you get no gas at all. All are pinned in, doubt they would stay in place if fired w/o one. The rifles have been checked for for headspace and pass on all 3 gauges. I will make the recomeended measurements and see if I need to flare the tube at the key end. To me it seems as if it's running out of pressure b4 the bolt has completly cycled. But if that isn't the problem, what other info would be helpful?

Do you mean the barrels accept all three gages?

Gas tube alignment would really be my guess.  (A bolt key too small for the gas tube would do the same thing.)  The extra resistance would definitely cause short-stroking.  Have you tried inserting the bolt/carrier into the upper by hand and making sure it slides smoothly forward as the key encounters the gas tube?  Likewise on extraction?

From the title of your post, I'm guessing that these uppers all worked fine before the installation of float tubes?  It seems odd that all 3 uppers would suddenly develop the same problem.

Float tubes themselves have NOTHING to do with the gas systems, although the manifolds can.

Nobody has asked for the dimensions of you barrel orifices, and that might matter. What is the orifice size?

Was the previous front sight housing or manifold sealed and these are not?  Some folks are using Loctite to anchor the front sight, and that seals them too. Are the manifolds and gas tubes leaking anywhere? They are allowed to leak a little, but if they are a loose fit, they will cost you functioning.

Are the ends of the gas tubes worn? This can cost you function too.

All FF tubes I have seen require replacement of the barrel nut.  Misaligning the barrel nut can cause your gas tube to bend left or right inside the upper receiver.  This can cause binding between the gas tube & bolt carrier key.  Voila: short stroking.

I'm still confused about how these problems developed, though.  Did they all work fine at one point, and then the installation of FF tubes caused problems?

You can mess up gas tube alignment with any method of barrel attachment. I would look at the tube for misalighnment and wear. If the wear is on either the left or right side, you need to check the barrel nut alignment. This still will not usually mess with stroking until the tube gets some wear on it...

Thanks for all the input.
2 of the 3 function flawlessly, the trick was to guntite the gas tubes to manifolds to form the seal good machining used to. Thanks for that tip. It also makes get the pins in easier.
I need to start another thread on the DPMS Bull Barrel upper soon.
THANKS AGAIN for all your help.

DPMS sent out a few barrels that the chamber were reamed too tight.  You may want to send the barrel back to them to have the chamber checked out.

My DPMS did the same thing you describe and it ended up being a tight chamber and a small gas port. They fixed it for free but it still pissed me off to the point that I will be using a different brand barrel on my current build.

This is a SS BB 1x8 24". And it has been a test of my patience. I have had that thing apart more times than I care to mention. And I believe that it is chamber related. There are longitudinal scratches on the empties from the crimp past the neck, and I was able to decrease them by power brushing the chamber, but not eliminate them. Do you know when the barrels were made? This one is about 18 - 24 months old, and has been vexing me since May 2003. I will call them and find out what they can do about this. Thanks for the input.

BTW, to all, the guntite at end of the gas block provided the addtional seal/baffle that you  get with from the front handgaurd retainer that butts up to the fr. sight housing/manifold.
I had NEVER added anything on any (either of them)upper I built prior to free-floating.

My DPMS barrel that I bought is less than a year old. I would call them and see what they say. I described the problem to them and they told me to only send my upper back and they fixed it within a couple weeks.

Call DPMS and ask for Dave. I know that I fucked with my gun for over a month trying to figure it out and it drove me crazy.

I have a DPMS upper that I too am having alot of problems with.  When I first put it together, I could hardly get the bolt to go into the carrier.  I know that with new gas rings and such it might be a little tight, but this was ridiculous.  Once I got it in, the bolt would turn about 1/3 turn in the carrier then it would start to bind (no cam pin, of course).  Then I found that the bolt would not move front to rear without ALOT of force.  Even after 70 or so single fire shots through it, it still would not move right, so "wearing in" was not the problem.

After reading the above about the gas ports/manifolds and tight chambers, I am wondering if this might be the problem.  I contacted Dave at DPMS over the phone once, and he suggested putting a larger caliber cleaning brush (I used a 40 brush) on a short section of cleaning rod, chucking it in a drill and cleaning the chamber with it.  He explained that DPMS sends out their new barrels with an anti-rust coating in the barrel and chamber and this could be the culprit with my problems.  I did as he suggested, took it out and fired it, and found that it made no difference at all.  It was still short-stroking.

Just last week I e-mailed DPMS again about the problem and they replied that I should send in the complete bolt and carrier, and they would check to see if there was a problem with it.  But after reading the above, I am starting to think maybe I should send in the whole upper and have them check all of it in one fell swoop instead of dragging all this out for ANOTHER couple of months.

I am getting rather frustrated with this whole thing.  I did not put out several hundred dollars on this upper for a "kewl” looking single shot or a kick-@$$ paperweight!  

Send the whole thing back.

I took it out after talking to Dave about Ammo selection.
When I use .223 ammo (except Win White box), it funtions fine. When I use 5.56x45 NATO ammo, it's a single shot. I'm going to sell the whole thing anyways. It was really just a "sample/project" while I am finishing my AGI course work. And it is to heavy for me carry, loaded or in a case. Exceeds my 10 lb. lifting restriction. But I wanted to sell it digesting all ammo. I may do the chamber thing with gunbrite abd the over sized bore brush, but from Dave said about ss109 pressures/case length and chamber walls, I may just list it with the DPMS recommended ammo choices. There is a listing of them some where on forum.