Just finished testing a new M4 flattop upper today.  The bolt carrier(not sure of origin) showed brass marks on the edges of the central ridge on the frotn underside (look at a carrier and hopefully this will make sense).

Is this normal?  I have two other uppers with their own bolts, and neither of them show this kind of brass rubbing on the carrier.  I don't think it affects anything, though I did have the  final round in an SA-80 mag get the bullet pushed in- hopefully that was just a mag problem as described in a similar post.  Any thoughts would be appreciated.  

Thanks in advance!

Dawg

Do you mean where the new/rough carrier is riding over the loaded ammo in the mags, and is pushing the ammo down in the mag as the carrier glides past on the reaward stroke?

That would appear to be the case, though some of it appears to be wear caused by the hammer- apparently the parkerizing on the bolt carrier sucks. I did verify the ammo misfeed was related to the mag (didn't have a anti-tilt follower in it)

Try some G.I. 20 rnd. magazines. It may not make a difference, but check the SA-80 magazine is not riding too high up in the lower. Compare it with your other rifles and measure the magazine protrusion with the upper/lower seperated.