After sending back two crappy kits, my son finally got a decent one.

Does anyone have any idea how to position the shroud and guarantee proper gap on the cocking support without a barrel? Trimming everything back to where it was from the factory seems to have put stuff where it needs to be but I can't really check the cocking support / bolt carrier gap without a barrel I don't think. IIRC the G3 barrel sits about 0.015 proud of the aft face of the trunion, so it would create a bit more gap than just putting the bolt right up against the face of the trunion. I think. Or maybe I'm being a pedantic MF, or a dumbass, or both...

ETA: I did make a dummy barrel that is a tight fit into the trunion, and it is working great to keep everything aligned, but it's not extending through the trunion and contacting the bolt face. I didn't trust any of the measurements I got enough to set bolt gap so didn't even try. I wasn't thinking about the cocking support gap though.

Hmmm. I trimmed back my cocking tube until the receiver could slide all the way into the barrel shroud and just touch the "ledge" on the inside of the barrel support. Is it supposed to be more complicated than that?

Maybe not. That's what I've done so far.

With my G3 build I had to space out the cocking tube a bit more than I had figured on to get proper gap between the cocking tube support and the abutment (think those are the correct terms). I remember being instructed to put a shim in front of the cocking tube support at the time. I was just thinking I might end up with less gap than I should if I set it that way with no barrel in there.

Maybe I should go see where it sits now. If that shroud is indexed off of the trunion then it probably has to work.

Speaking of the cocking tube...I guess I'm now just realizing that there is supposed to be a sleeve that actually goes into the receiver? And this sleeve was mostly destroyed in the demill process. How are you guys handling this? Make a new one?

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Edit: i went and double checked the tutorial on weapons guild. They appear to have ignored this detail.

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I think most just mill it off flush. It's shorter than a G3 sleeve I think. Seems like I found a pic of one somewhere and it was only 1/2-3/4" or so but it's hard to know if that was really a full factory cut. One guy in one of the other various threads on 21 builds carefully removed the outer tube up to the weld and retained what he had of that sleeve to help with alignment.

One other guy over at WG noted that his carrier was hanging up on that transition. He had some other alignment issues that made him frustrated enough he threw in the towel though, so there was probably more going on with his.

I have a mandrel that I turned that fits snug through the receiver and is stepped down to fit snug in the cocking tube as well. I think it's 0.810 through the receiver and 0.750 through the cocking tube. It's aluminum and I was planning on using it as a weld backer/heatsink as well. I cut my tube off flush and am hoping that keeps it lined up well enough without that extra bit of thin tube. Just might be enough of a "bump" if things aren't lined up well to mess with the bolt travel though. I hope I'm not building a long sanding/grinding tool to deal with it next week.

ETA: Page 18 of the 21 build thread over at WG has some discussion and pics of a couple other ways of doing it. 21HK is the guy who managed to retain a bit of that inner tube.

That junction there did end up causing me some trouble. I had to redo it a few times to get it just right before the bolt would cycle smoothly. Ramming that alignment rod in there one last time did the trick. Finicky bitch.

I’m still nervous about the cocking piece gap. I think mines too large as is. If it’s like the G3, the barrel sits ~0.015” proud of the trunion. Then there is the bolt gap that’s probably another 0.015”. I’m thinking the gap should probably be close to zero with no barrel in it to put it at 0.030 when it’s done. Maybe it’s not that critical but I’ve had a PTR where it was too big and it’s a bitch to cycle.

I may just wait until I have a barrel before I weld the shroud I guess.

My bolt gap is zero with no barrel installed

Thank you. That is reassuring. It took me trimming it back a few times before I got it there but that's where it's at now. I stopped short of welding the aft ring of the shroud yesterday. The shroud is aligning really well with the dummy barrel and bolt is cycling smooth so I think I'm getting there.

I probably should have ordered a barrel or two from cryo but decided to wait on the RCM barrels. Man I hope they hurry up and get them shipping before Oct.

ETA: Didn't read very carefully. I think the bolt gap will always be zero with no barrel. I was talking about the gap to the cocking tube support. I think you want just a bit there with barrel installed and proper bolt gap to prevent the thing from battering itself to death as the bolt gap shrinks over time. If you set it too large though there won't be enough camming action to pop the bolt out of battery. I had a gun like that and it was a PITA.

That makes sense, I couldnt tell you what my cocking tube support gap is because I wasnt looking for that last time I had everything fitted together. I will check that next time I assemble it.