I've got a problem that I cannot seem to figure out.

New build, BCM 16" 410SS barrel, BCM bolt, JP carrier, VLTOR upper.  Barrel and receiver have M4 ramps, they line up fine.

The problem is a failure to feed.  The new rounds get caught up on the feed ramp.  I've never had a problem with BCM barrels and have quite a few of them.

Empty brass kicks out just fine and will lock back on empty mag every time.  Gas is not an issue.

When the gun is dripping wet it is usually fine for a while.  After about 80 rounds it goes tits up.

I've tried three different brass cased ammo types.  

I've swapped complete lowers that have been working for years.  

I've swapped complete BCG's.

I've tried 4 different mags that have been working flawlessly in other guns for 2+ years.

Hand cycling rounds is a train wreck.  Usually about 50% hang on the feed ramps.

Loose magazine catch? Tighten the magazine catch a turn or so.......
Maybe it is allowing the magazine to tilt forward, causing an odd angle for the round to feed from the magazine.

Receiver extension have sharp edge or burr? Something sharp catching the brass as it tries to feed?

Pictures please.........

It's not the mag catch, I've tried three different working lowers.

I just swapped out the upper receiver with another VLTOR.  Same problem with most of the rounds hanging up on the feed ramps when hand cycling.  The barrel must be bad.