You have narrowed it to the upper assy.Try your upper on your buddy's lower,but switch bolt carrier groups.If the different bolt carrier works,look close to see if the staking of the top screws has pushed some metal up that is dragging.If so,dress it off flat.
Try swapping charging handles. Look real close at your forward assist to see it is fully retracted.
You will get it.
I think probably ,unless you bought some super cheap knock off parts,the upper and lower receivers will be to spec.If it is a carbine stock,you need a carbine length buffer and spring.
What stock and tube are you using?
What stocks and tubes were on the alternate lowers?
Take your upper assy off,pull the bcg out .Look at how the gas tube lays in the upper.Is it centered?The one place you can mess up an "alignment" is by not aligning the hole for the gas tube when tightening the barrel.
Another possibility,were you using proper tools or make do tools when holding the receiver during barrel nut torquing,etc?Any oppurtunities to crush,twist or otherwise distort the receiver?Is this a surplus parts kit ?
Last edited by HiBC; March 14, 2012 at 05:31 PM.