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If you bought an HK-21 kit get in here! (Now a build thread…) (Page 15 of 17)
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Originally Posted By Kuraki: Do we know now long before Rim Country has more? I don't know whether I should order a Cryo now or wait. FYI: I'm still waiting on Cryo to ship mine. I ordered when they were announced. I was on the RCM notification list and he contacted me when they were available. |
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Originally Posted By Pro_Patria_431: FYI: I'm still waiting on Cryo to ship mine. I ordered when they were announced. I was on the RCM notification list and he contacted me when they were available. I don't see a way to sign up for that for subsequent batches, though I did just send them an email to see if that's possible. |
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Originally Posted By Kuraki: I don't see a way to sign up for that for subsequent batches, though I did just send them an email to see if that's possible. Originally Posted By Kuraki: Originally Posted By Pro_Patria_431: FYI: I'm still waiting on Cryo to ship mine. I ordered when they were announced. I was on the RCM notification list and he contacted me when they were available. I don't see a way to sign up for that for subsequent batches, though I did just send them an email to see if that's possible. An Email is all I did. The confirmation was the availability notice a couple of months later. |
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I received my Cryostructure barrel today. I also already have a RCM barrel. Here are my comments: The RCM barrel does not come with the quick change handle assembly. I ordered the Cryo barrel (and paid extra) for the quick change handle assembly. It did not come with the handle assembly. A hand written note says those parts are not available yet, but will be and he wanted to go ahead and ship to avoid further delays. The RCM bolt gap is .018". The Cryo bolt gap is .013". The RCM has a black nitride type finish with their big logo etched on it. The Cryo has a nice dark gray parkerized finish with no markings (other than I did request a custom serial number engraved and they did a nice job with that). The RCM barrel does not have the pin at the rear face of the barrel that interfaces with the hole in the front face of the bolt head (this pin prevents firing if the barrel is not locked). The Cryo barrel does have this pin. The RCM shipped basically on time The Cryo shipped several months late The RCM barrel is not chrome lined The Cryo barrel is chrome lined Both barrels drop in and rotate very smoothly. Cryo includes a nice gift certificate for a large discount off of a specific product they sell. They also include a note that states they have a lifetime guarantee on all of their parts. I have not fired, or even chambered a round, in either barrel. So, I have no idea about how they function or their accuracy. That's all I have for now. |
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Bump!!! Mine is officially all done. My welding got real real good.... towards the end of the build. So not the greatest looks wise around the receiver section, but happy none the less. I also did some electro etching on the side with the kids, and they loved watching that process! Just made up an SN with the last three of the gun, and a type and added it to the side. Also if you don't have some of these M13 started tabs, get some!!! They are awesome! https://www.apexgunparts.com/starter-tab-m13-links-7-62x51-5-pack.html    
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Originally Posted By TZLVredmist: Bump!!! Mine is officially all done. My welding got real real good.... towards the end of the build. So not the greatest looks wise around the receiver section, but happy none the less. I also did some electro etching on the side with the kids, and they loved watching that process! Just made up an SN with the last three of the gun, and a type and added it to the side. Also if you don't have some of these M13 started tabs, get some!!! They are awesome! https://www.apexgunparts.com/starter-tab-m13-links-7-62x51-5-pack.html https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155811787-3531234.jpg https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155726610-3531235.jpg https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155503476-3531237.jpg https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155556973-3531239.jpg  It looks fantastic.
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Originally Posted By TZLVredmist: Bump!!! Mine is officially all done. My welding got real real good.... towards the end of the build. So not the greatest looks wise around the receiver section, but happy none the less. I also did some electro etching on the side with the kids, and they loved watching that process! Just made up an SN with the last three of the gun, and a type and added it to the side. Also if you don't have some of these M13 started tabs, get some!!! They are awesome! https://www.apexgunparts.com/starter-tab-m13-links-7-62x51-5-pack.html https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155811787-3531234.jpg https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155726610-3531235.jpg https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155503476-3531237.jpg https://www.ar15.com/media/mediaFiles/47383/PXL_20250502_155556973-3531239.jpg That looks great! |
Joined:
Jan 2023
Posts:
42
EE: 100% (2)
NV, USA
Man...just read through all 15 pages and I WANT one of these kits. You guys make it hard to be financially responsible  ...never even considered an HK belt fed until now lol.Originally Posted By rugerdog: Which kit from Centerfire is the best for building on of these kits? I see they have a few at different prices. -Thanks Wondering the same thing - are the type B cuts worth the extra $$$ or will builders just be more annoyed with having to tear off the old receiver. Seems like the Type B cuts are less risk of damaged trunion/shroud/etc. though. |
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Originally Posted By xtremegibs: Man...just read through all 15 pages and I WANT one of these kits. You guys make it hard to be financially responsible ...never even considered an HK belt fed until now lol.Wondering the same thing - are the type B cuts worth the extra $$$ or will builders just be more annoyed with having to tear off the old receiver. Seems like the Type B cuts are less risk of damaged trunion/shroud/etc. though. I haven’t heard of any of the kit sellers not taking back and replacing a kit if the trunnion/shroud was damaged during the demil process. I’d go with the cheaper option personally. |
Joined:
Dec 2022
Posts:
77
EE: 0% (0)
UT, USA
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Originally Posted By strikeone: RCM 21Barrels going to Nitride on 5-8-25. Another 300 on OP 1. Getting close Nice, Just picked up a 21 kit for a good price and been obsessively checking to see when they come back in stock. Also will there be more 36 degree locking pieces soon? |
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Headed to a 1000 yd range on Sunday to stretch out the legs on the .50 and decided I’m gonna take the 21 too. I’ll do some accuracy testing with the Cryo barrel and run it out to 1k for shits and giggles. Will post some reviews and videos if I remember to take a few. Attached File |
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Originally Posted By cms81586: You’d need to ream the chamber flutes too. Yes, I'd make a scraper to do it. I'd try casting an existing chamber with Cerrosafe to get the geometry, which might not work and why a stub would be preferable so I could section it if necessary. |
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Originally Posted By Mrf2: What is the consensus about the best Mk48 style belt box adapter? I see that cryostructure had one (now OOS) and several people have talked about making some, are there other options? I am messing around with an old MG3 feed tray that I have cut down for only the assault drum setup. I plan to make it a bolt on adapter for the base so the gun will accept the more modern feed drums. Will post pics when further along. These plastic HK drums can be found real reasonable in new condition. https://www.robertrtg.com/mg3-assault-50rd-drum-hk-new-old-stock Just another option |
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Anybody here know how to determine if the buffers in the kits are good? Also, anybody who has built a kit with the original buffer, how is it holding up? I have read these 21 literally beat themselves to death. 21e buffers and gone, so is it worth building one of these with the original buffer? I don’t want to tie up that much money in a short lived gun if the system was not good to begin with. Thanks |
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Originally Posted By titan7: Anybody here know how to determine if the buffers in the kits are good? Also, anybody who has built a kit with the original buffer, how is it holding up? I have read these 21 literally beat themselves to death. 21e buffers and gone, so is it worth building one of these with the original buffer? I don’t want to tie up that much money in a short lived gun if the system was not good to begin with. Thanks I want to say most of the wear and damage comes from running them with out of spec rollers and improper builds that lack the proper cocking tube/carrier gap. Originals were shorter lived than 21E’s but it’s not a gun you’re going to wear out without spending several times its value on ammo. |
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Originally Posted By titan7: Anybody here know how to determine if the buffers in the kits are good? Also, anybody who has built a kit with the original buffer, how is it holding up? I have read these 21 literally beat themselves to death. 21e buffers and gone, so is it worth building one of these with the original buffer? I don’t want to tie up that much money in a short lived gun if the system was not good to begin with. Thanks Drop the locking piece to a 40 degree, install a new production G3 heavy buffer and replace the recoil spring. Gun will shoot extremely soft and you wont rip anything apart. |
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Originally Posted By titan7: Will a G3 heavy buffer will fit without moving the cocking notch forward. The "TOP HAT" heavy buffer is what these early kits came with no "cocking notch" mods *required* not sure about the G3 heavy buffer but i think i read on HKparts that your stock may need modifaction for it to fit. Your HK21 kit SHOULD have come with the TOP ONE one though. I am not sure how much of a difference the Top Hat version would make over the G-3 heavy buffer or "vice versa" All i know for sure is DO NOT use a normal G3 buffer with these - it will destroy the back plate of your 21 Ferrule from impact. my 2 cents - YMMV. normal G3 buffer in one of these kits. This is how one of my kits came that had a standard G3 buffer installed. Attached File Completely "pushed & pulled* the recoil spring mounting area till it gave up the "ghost" whorped the area and destroyed the entire ferrule. BTW Jared at Desert Yankee can fix it PERFECTLY but time consuming and not exactly cheap. (You can see the BEATING that standard buffer was taking just looking at the finish wear and my kits where not in worn out condition) |
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Originally Posted By 7insert: Drop the locking piece to a 40 degree, install a new production G3 heavy buffer and replace the recoil spring. Gun will shoot extremely soft and you wont rip anything apart. If your kit came with a braided recoil spring i see no need to swap it out (imo) otherwise: THIS................. |
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Originally Posted By titan7: Will a G3 heavy buffer will fit without moving the cocking notch forward. Originally Posted By titan7: Will a G3 heavy buffer will fit without moving the cocking notch forward. Yes Originally Posted By barbcue: If your kit came with a braided recoil spring i see no need to swap it out (imo) otherwise: THIS................. No other need other than the springs being older than most people on this website While the rest of the springs are similarly aged, the rest of the springs arent working to soften the blow of a violent blowback gun. |
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Originally Posted By 7insert: Yes No other need other than the springs being older than most people on this website While the rest of the springs are similarly aged, the rest of the springs arent working to soften the blow of a violent blowback gun. @7insert Are they durable enough (the G3 heavy buffers) to use in the 21? I have a friend who bought a stack of them years ago and he offered me one if I wanted it. |
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Originally Posted By cms81586: @7insert Are they durable enough (the G3 heavy buffers) to use in the 21? I have a friend who bought a stack of them years ago and he offered me one if I wanted it. Absolutely the case, especially with the reduced locking piece and an upgraded spring you should be installing first. Couple thousand live and blanks with that setup so far and no signs of dents/things coming apart. |
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Originally Posted By 7insert: Absolutely the case, especially with the reduced locking piece and an upgraded spring you should be installing first. Couple thousand live and blanks with that setup so far and no signs of dents/things coming apart. Is this the one you’re talking about? Or the bigger one that comes in the SG/1 stock? Attached File |
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Originally Posted By cms81586: Is this the one you’re talking about? Or the bigger one that comes in the SG/1 stock? Nothing for scale there but yes that looks like an oldschool G3 heavy buffer FWIW there are newer heavy buffers can be found in the same format as a standard length G3 buffer. HK Parts new production heavy buffers, bill springfield and DPM Systems of greece all produce small heavy buffers. https://www.dpmsystems.com/en-gb/heckler-koch-g3-buffer-assembly |
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Originally Posted By 7insert: Nothing for scale there but yes that looks like an oldschool G3 heavy buffer FWIW there are newer heavy buffers can be found in the same format as a standard length G3 buffer. HK Parts new production heavy buffers, bill springfield and DPM Systems of greece all produce small heavy buffers. https://www.hkpro.com/attachments/img_2588-jpeg.432133/ https://www.dpmsystems.com/en-gb/heckler-koch-g3-buffer-assembly So I asked my buddy to look through his email. They’re HK made MSG-90 buffers. I’m still going to use it. Anything is better than the buffer currently in the gun (original FMP that does still have give….but not much travel at all. |
Anyone get a barrel from Cryostructure recently? I ordered a 17.7" without hardware on the 3rd. At the top of the product page they say it's "In Stock" and will ship in 48 hours... Then in the description it says  I hate updating my own website so I can empathize with the discrepancy, and I'm not in a rush so if it's going to be a wait that's fine too. It'd just be nice to know one way or another. |
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Originally Posted By Kuraki: Anyone get a barrel from Cryostructure recently? I ordered a 17.7" without hardware on the 3rd. At the top of the product page they say it's "In Stock" and will ship in 48 hours... https://www.ar15.com/media/mediaFiles/125898/Screenshot_2025-07-28_145457-3596024.png Then in the description it says https://www.ar15.com/media/mediaFiles/125898/Screenshot_2025-07-28_145512-3596027.png I hate updating my own website so I can empathize with the discrepancy, and I'm not in a rush so if it's going to be a wait that's fine too. It'd just be nice to know one way or another. Still waiting on mine I ordered a year ago. |
I mean…all that work to make it an overgrown G3. WTF…. Sort of a pointless accessory I’ll probably never use much but it’s neat to have thanks to a member here. Attached File Attached File Attached File Attached File |
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Originally Posted By Kuraki: Anyone get a barrel from Cryostructure recently? I ordered a 17.7" without hardware on the 3rd. At the top of the product page they say it's "In Stock" and will ship in 48 hours... https://www.ar15.com/media/mediaFiles/125898/Screenshot_2025-07-28_145457-3596024.png Then in the description it says https://www.ar15.com/media/mediaFiles/125898/Screenshot_2025-07-28_145512-3596027.png I hate updating my own website so I can empathize with the discrepancy, and I'm not in a rush so if it's going to be a wait that's fine too. It'd just be nice to know one way or another. Last I spoke to Paul his new machine is up and running and he is getting barrels done…and has a bunch more blanks coming in. His chrome shop seems to be the bottleneck. |
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Originally Posted By cms81586: Last I spoke to Paul his new machine is up and running and he is getting barrels done…and has a bunch more blanks coming in. His chrome shop seems to be the bottleneck. Well I can relate to that. Platers of all kinds are always a bottleneck for my work when it's required. |
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Originally Posted By Kuraki: Well I can relate to that. Platers of all kinds are always a bottleneck for my work when it's required. @Pro_Patria_431 @kuraki HK Parts just listed RCM 17” barrels in stock 15 mins ago. https://hkparts.net/rifle-smg-parts/rcm-hk21-barrel-17-7/ |
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Originally Posted By cms81586: @Pro_Patria_431 @kuraki HK Parts just listed RCM 17” barrels in stock 15 mins ago. https://hkparts.net/rifle-smg-parts/rcm-hk21-barrel-17-7/ oops I didn't read. Went to RCM page not HKP. ETA: Got one, thanks for the heads up OP! |
If you bought an HK-21 kit get in here! (Now a build thread…) (Page 15 of 17)
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