I came across a problem last night when going through the troubleshooting checklist on two new builds.  The two builds behave the same way.  One upper was built by me, the other by a local AR smith.

Problem

With the bolt locked back & chamber empty I insert a full magazine, ensuring it's locked in place by the mag release.  I push the bolt release, and as the bolt moves forward it begins to strip a round from the mag.  It then gets stuck as the tip of the round gets hung up in the barrel extension lugs.  See pic below.  



Rifle stats

Build #1
RRA upper, no M4 feedramps
Bushmaster 16" Superlight barrel assy
RRA b/c/ch
Stag complete lower
RRA Fiberite 6-pos telestock with telestock buffer & spring
Colt M4 handguards
Standard FSB
USGI mags that have not presented problems before
XM193 and South African ammo behave identically
All parts are brand new
Headspaced OK with USGI field gauge
built by me

Build #2
RRA upper, no M4 feedramps
DPMS b/c/ch
Bushmaster 20" HBAR
Stag complete lower
A2 stock with standard buffer & spring
YHM FF rail
RRA railed gas block
USGI mags that have not presented problems before
XM193 and South African ammo behave identically
All parts are brand new
Headspaced OK with USGI field gauge
built by AR smith

The mags & ammo that are causing these problems work fine in an all-Colt 6530, an LMT M4 upper on a Stag lower, and a Colt M4 upper on a Stag lower.

Thoughts

I understand that this can be caused by mix & match parts where the upper has M4 ramps but the barrel does not (as the tip of the round will get caught under the edge of the barrel extension), but that's not the problem here.  The tip of the round actually gets wedged between the lugs themselves.

I've taken a look at the FAQs (No. 1 in particular) and I don't seem to see anything describing this exact problem.  The closest is a short stroke, but that's not the case here since my problems are with the first round in the magazine.

One possible problem could be that the barrel is too far from the mag, but the barrel nut on the upper I built is tight, and I have to assume that the barrel nut on the upper built by the smith is tight too.

Any ideas?

Son of a gun, just found this thread.

I'll lay even odds that if he dropped the mag, the bolt would still be locked/fozen into the same position.

My guess is that the hammer is grabbing the firing pin collar, and that the bolt it's self is the part that is hanging up in the action.

*cough*

I spent a few minutes filing last night but haven't noticed any difference.    I didn't take off too much material, but there's a bright line where I can see that the sharp edges have been rounded off a bit.  

When I take out the mag and b/c/ch and run the tip of the cartridge over the lugs I can feel resistance as I push the round towards the chamber.  Just to see, I did the same thing on my Colt and feel just about the same resistance - but the rounds aren't hanging up in the Colt.  Same ammo, same mags.

Guess I'll keep filing (though not too much) until the rounds feed smoothly.

Any other suggestions?

Quoted: I'll lay even odds that if he dropped the mag, the bolt would still be locked/fozen into the same position. My guess is that the hammer is grabbing the firing pin collar, and that the bolt it's self is the part that is hanging up in the action.

The same thing happens with two different bolt designs - a DPMS with an unshrouded firing pin and a RRA with a shrouded firing pin.

If it were the pin shroud hanging on the hammer, it would happen with or without a round in the well.  I have had the same problem numerous times, I've had to polish out the feed ramps until it feeds correctly.  It's not that big of a deal as long as you try to hold the same contour as the existing feed ramps, just pretend your dremel tool is a round and aim for center of barrel bore.  Dremel and others make a cutting tool that is the same approximate shape as a .223 bullet, that is what I use.  But any small, fine cutter for a dremel should work as long as you take your time, cut just a little off each side at a time and then polish with a "kratex" or dremel polishing wheel.  Don't be timid, you can do it.  I have accidently opened up the ramps (I was much younger and dumber) to look like it was cut for a .308 and believe it or not, it worked perfectly.  It appears from the photo that height is not a problem, just the width.  I've had more problems with Bushy's barrel extensions than from any other manufacturer.