When pulled back, the carrier will just sit and you have to tap the cocking handle like the FA on an AR to get it to slam home.  If you don't give it a really good whack, it will get caught further up just as it starts to strip a round.

What gives?

This is a Polish underfolder built on a GTUSA 100% receiver.  Therefore, it's not 100% my fault and I know you guys can help

I am pretty sure the bolt isn't getting caught on the center support.  I've read that people file the top of the center support flat for just such a problem but I'd rather not do that.  THE FRONT of my center support is actually flat.  Did GTUSA mess up on this one or is it supposed to be that way?

I feel a lot of resistance at the hammer as well.  Could it just be the hammer?  It's a G2 double hook FCG...

how much side to side slop does the carrier have?, it might be too tight, if it is, take a 1x2 board and place it between the rails and twist, with a GT receiver it will take some effort but it will give the carrier the clearence(slop) it need to slide freely.

I have done a bunch of these...

The main issue here is usually the UNDERSIDE of the upper rail. They tend to use dull tools on the cuts, and there is almost always a large burr there.

What happens is that you start trimming the rail itself, but the burr causes the carrier to still hang up. I figured this out one time when I trimmed the rail, and then measured good clearance between it and the carrier....I felt under the rail and HUGE burr. Like I said, I have found this in several receivers from GT.

So, remove the burr FIRST so that you don't take too much off the rail geometry or have to work with possible receiver twist. Work with the FCG and bolt OUT of the carrier so you isolate the movement of the carrier in the receiver.

If you want to know where something is binding up, take a marker and mark up the carrier...put it in the receiver and see where the marker is scratched...then work from there.

good ideas!  I'll check the underside for burrs first and then report back...

i take it that filing off the burrs with a long file will work. No dremel needed here, right?

Quoted: good ideas!  I'll check the underside for burrs first and then report back... i take it that filing off the burrs with a long file will work. No dremel needed here, right?

Correct, but I find it much easier to use a heavy duty cut-off wheel on the Dremmel Tool of the Gods, and very carefully use the edge of it on the underside of the rail. It helps you get that 90 degree angle you need. It's hard to get a decent file in and under the rail...

check to see if it is the hammer...Sometimes it will hook under the bolt carrier....
Check your side play on the carrier against the rails

I had the same problem with my GT (Armory USA) receiver.  The guy who mentioned the burrs is absolutely right, my rail had such bad burrs that it cut my finger like a razor, so be careful.  A GT rep told me that his builders simply hammer with a soft hammer on the carrier wherever it binds, for what it's worth.  I tried that without luck.  Finally, gently filing away at the burrs with Dremel and file and even a bit of filing of the inner surface of the rails did it.  Just do a little at a time and keep rechecking.  Removing the FCG will help make sure you don't confuse the binding from the rails with that of the hammer.  The GT rep also told me that the center support flat side is supposed to face forward...
With that I will resume celebrating the removal of the barrel pin from the Romanian kit!

TheNorm was right in this case!!!  

You are the man!  I just took a file and ran it across the undersides of the rear two rails and each one coughed up a nice long thin edge that was just begging to come off.  The front two were harder to get to but I was able to get a good amount of filings from them.  

After I finally and hopefully permanently secured the barrel assembly into the receiver, I came in the house, inserted the carrier, spring and installed the dustcover, then insterted a mag and proceded to rack away, screaming woohoo like an idiot and got yelled at by my fiancee.  

Oh well, at least it works.  

vetty vetty nice...

<little chuckle like a mad witch doctor...>