I'm moving a barrel from an A2 Bushmaster upper to a flattop Del-Ton upper. I've got the barrel off the A2 upper, but I'm having a bit of trouble getting it installed in the new flattop upper.

I can't get the snap ring (at least I think that's what it's called, see picture) to line up so that the gas tube can be installed. The ring doesn't start to tighten until the notch gets to about  9 o'clock (as pictured). I can really crank on it and get it to about 7 o'clock, but I can't get enough force on it to rotate it beyond there (I'm practically hanging off the wrench just to get it that far). If I loosten it until the slot lines up at 12 o'clock again, the barrel will rattle.

I've put a little bit of grease on the threads, and there doesn't seem to be any dirt on them.

Any suggestions?
                                         

(I know, it's not a great picture)

Oh, and the barrel is a bushmaster 20" HBar, the new upper is a Delton flatop (w/o T-marks or M4 feed ramps), and I'm using a DPMS wrench and upper receiver block.

Leave the barrel nut alone. Now, with a 1/16 inch punch insert it in one of the two holes in the snap ring and rotate the snap ring gap to the 12 o'clock position. Again, this is an operation that is INDEPENDENT of doing anything with the barrel nut once it is torqued down. E-mail if any questions. Merry Christmas. Charles the Gunsmith.  

Ditto with 1911,

The delta snap ring/C ring will rotate independent of the barrel nut (just need to spin it from the receiver side of the delta, and not try to spine it threw the front side since the barrel nut spines will block you).

But once you do get the delta ring hole and the C ring slots into the correct position, you need to check to make sure that the upper spines of the barrel nut are adjusted/indexed correctly to allow the gas tube to pass threw the top two barrel nut spines without binding.

Thanks guys. It's all together now.

Time to drop in the carrier without the bolt and make sure that the gas tube is lined up correctly with the key.

Also, don't forget to bore sight the barrel with the sights center.  Last thing you want is to go to the range and find out you canted the barrel on install.