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.

I know a lot of us are waiting around on barrels and haven't gotten too far into the feedbox deliberations yet. I had been planning on using tribe tactical belt boxes that interface with the M249 dovetail (Mk48 nutsacks). HDD sells an adapter bracket/weldment. I have one and made another using it as a reference.

Tribe has been pretty well OOS, especially for the type II nutsack. I've had some email contact with him and it sounds like he's at least contemplating another run of the nutsacks. Sounded to me like he was wanting to gauge the demand before making another run.

I'm not trying to be a saleman for this guy but I would sure love it if he did another run. If anyone else is interested it would likely help things along if you sent him an email.

Here's what the type II nutsack looks like:


https://tribetacticalsupply.com/mk-48-bfm-type-2-nutsack/

I bought 3, one had the locking tab located too far out and when the box is fully inserted the ammo box won't lock into place. Other than that, they are well made items. I'm going to tey to re-locate the locking tab myself.

I ordered a couple of the nut sacks and the HDD weldments. I am thinking about a couple more.

I’ve got my second one close now but still have an issue with the recoil assy to work out. This one was going to be a 51B but I decided to add the feed mech to a G3K build I already had going.

I wish I knew when RCM was going to release more barrels, and new trunnions and bolt heads.. The suspense is killing me!!! That is the only thing holding me up from sending my kit off.. both of my bolt heads are OK… not great.. So I would rather start with all new components..

Hopefully soon..

I do appreciate them making these unicorn parts.

Thankfully i'm stacked with FMP barrels and a few HK barrels.  More than i'll ever be able to burn out with my crappy ammo budget.

I was able to snatch up one of the first batch of RCM barrels. I wanted a couple but with my ammo budget, it will probably last a while.

Part of the reason I built the one above was to get around the lack of extra barrels. That G3K barrel is pretty light though so it will be interesting to how it holds up.

PO is playing lost and found with the cocking handle support I need so I may have to wait until next weekend to test fire it.

PSA: the beltfeds use a slightly larger recoil rod that doesn’t quite fit into a G3 support. It tends to jam up the action at the very back of the recoil stroke. I thought I had the pack(ejector) height messed up but it’s smooth as butter with a G3 recoil rod. Just not quite enough ass to reliably strip rounds.

I think the 3K is where the beltfed platform should be.  The swap barrel is cool for a collection example I am working up but for funsies the G3K looks to be the ticket.

If you’re really shooting fullauto, the QC barrel is a must. The OG 51B used a 21 barrel turned to fit into a G3 trunion to give it some longevity.

If you like the 3k, chop and weld the shroud and build a 21K.

If you’re only shooting semi it won’t matter.

21K is already in the works. Chopping those shrouds down is a PITA though, I have a ways to go on that one.

I kinda wanted this one to be a lighter weight gun but the G3K barrel is a really skinny thing. We’ll see. I’ve actually had pretty good luck with RCM’s CHF barrels on other sear hosts.

It would be interesting to make a G3KB with a sleeper removable barrel. Use a 21 trunion. Use a skinny profile 21 barrel with the handle removed, mod the latch to be low profile. Weld the front sight to the cocking handle. Use a wide handguard.

I’ve been wondering about the cut down 21 barrels. I think the 21 and G3 trunions have the exact same ID but the 21 barrel is a tiny bit smaller. You would either need to sleeve the trunion, or cut the back off the barrel and then rechamber it right?

One of these days I’m going to experiment with trying to electroetch chamber flutes. That would open the door to a lot more roller locked shenanigans.