Today i built an m4 upper.  After i was done assembling it i placed the bolt carrier in and it would not go all the way forward to clear the back of the upper.  I placed 2 different carrier in and they worked.  I then took this defective (or so i thought) carrier and placed it in the 2 guns in which i "borrowed" their carriers to see if they fit.  It went in smoothly
can anyone explain this to me?

Gas tube thingy on top of carrier could be wigged out.

thats what i was thinking, but it worked in my colt m4 and my SPR upper perfectly.  The colt M$ and SPR bolts slid into this upper perfectly as well. I cant seem to figure out how this gas tube and this carrier are in spec but just dont work together

i would think its in the gas tube or the gas tube thingy on top of carrier or the bolt face isnt seating into the chamber..dunno..if ya ever figure it out, i would be curious as to why this was the case..also does the "T" handle play a role in any of this. good/bad or otherwise

Just to let you know. We took the key off the bolt carrier. The gas carrier key would not slide onto the gas tube. But it slid onto other tubes in other guns. He tried bolts from two of his other rifles no problem. He has that carrier and key in one of his other rifles no problem.
I thought maybe a burr but there wasn't one on the tube or the key. The key was torqued and staked down. The gas tube isn't bent. The only thing I didn't check. If it was a gas tube for a carbine.

die-tryin we tried it without the charge handle and bolt in the carrier and still no go. The bolt and carrier slid in all the way without the carrier gas key on.
Two other Bolts work and that bolt worked on your other rifles. I looked in the manual and the book I got from the class. Can not find a thing. Let me know what you find. If nothing call me durning the week and we could get together. I will try some of my bolts. I also have some gas tubes lying around some place I think.

[img]http://images.animfactory.com/animations/machines/tools/calipers_open_close_md_wht.gif[/img]

i just tried it in a 3rd rifle and it worked there.  I think i will just swap the carriers bwteen the m4 and this 3rd rifle