took my new toy to the range today.  i could rarely get 2-3 shots off without a failure to feed.  the bolt would stick about halfway up the round without it having been lifted from the mag (riding overtop the bullet, pinching it in and holding the bolt open).  the barrel is a BM superlight, on an A3 upper, old used bolt and carrier i pieced together, just assembled yesterday by a friend who is an armorer.  
basically, i'm just wondering if it could be anything but the mags.  problem is, i was stupid enough to only bring two mags, neither of which i had tested in another AR yet (DOH!), so i can't pinpoint the problem, however i'm sure its mag related, yes?
thanks for the help!

Provided the empty case is being extracted and ejected correctly...

and the bolt locks open on the last round everytime...

It sounds like the next round is poping out of the mag too early.  Use USGI mags.

both were USGI mags...like i said i hadn't used them previously so i can't say for sure that they would work in another gun either.  wish i had had the foresight to bring some tried and true mags along...probably won't have another chance to test it for a week or so which is why i was fishing for opinions here.
thanks

So if I read this right, the bolt is riding over the top most round in the mag without picking it up to feed it...right?

Sounds like either the rounds or the follower is hanging up in the mag or the springs are very weak....

Check for short stroke as OG "provided", if I'm reading your post correct that is what is happening.