I'm puzzled when comparing description of the problem with the pictures.
The threads shown, with the small pin poking out (which makes the little ratcheting noise when turning the barrel nut), seem to go to the same depth as in mine, which is to say the threading goes well beyond where they need to go, as the barrel nut just doesn't need to go that far into the receiver. The little pin is located in the same place, and should end up at about the middle of the ridged area at the end, beyond the threaded part of the barrel nut.
I suppose it's possible that the little pin is actually small enough, or the holes in the barrel nut used for accessing the barrel nut retaining spring (a circle of spring) with a small punch to remove it from its interior groove are large enough, that the ratchet pin is falling into one of those holes... stopping the barrel nut from turning before it's fully tightened on the barrel flange. But that would then jam, making it impossible to remove the barrel nut without removing the small grub screw, spring, then somehow fishing out the little ratchet pin, so this seems very unlikely.
I don't understand the bendy looking loop of wire around the barrel flange would help, considering that all other dimensions and threaded depth seem to be correct. Is it the spring clip which was supposed to be mounted in the barrel nut groove, retaining the barrel nut such that it can't slide past the slightly enlarged portion of the barrel just ahead of the flange? It looks too large in wire gauge to be that, though perhaps that's an illusion. The wire loop otherwise looks about right for that job. Mine measures 1.3mm in wire thickness, about 0.050". It is put in place by placing the loose barrel nut onto the barrel, within less than 1/2" of the tapered flange, with the spring clip already there ready for it. Then push the spring clip carefully into the taper at the back of the barrel nut with two small punches, one on either open end of the spring, holding the barrel nut firmly. Once the spring clip tips drop into the groove, slide the punches along either end of the spring around the barrel, towards each other, until the whole clip springs out into the barrel nut groove. Then the barrel nut is properly retained on the barrel, with about 1/2" of play forward and back.
I've stared at the images a while and can't see any other issue. Of course if that is the spring clip, then it is very bent, and needs to be straightened before it will go into the groove. Or you could cut almost a full circle (leave about a 1/8" gap) out of the same sized normal compression spring, clean up the ends with a file or Dremel burr, then follow the above procedure to install it. It's not a super important part - there's nothing wrong with the barrel nut coming all the way off - but it probably shouldn't be loose like that to tear up the aluminum taper of the barrel nut. You want full contact between the two tapers.
