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If you bought an HK-21 kit get in here! (Now a build thread…) (Page 16 of 17)
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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/ Thanks, Cryo sent a shipping notice last night.  I got the shop freed up, and can start working on fixing #1 and starting #2. |
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Originally Posted By Pro_Patria_431: Thanks, Cryo sent a shipping notice last night. I got the shop freed up, and can start working on fixing #1 and starting #2. What issues are you having? I was about to put together a post with some tips and tricks I wish I knew during my build. |
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Originally Posted By cms81586: What issues are you having? I was about to put together a post with some tips and tricks I wish I knew during my build. My biggest issue was my lack of welding skills.  I got it tacked up, and it worked, but I got carried away doing the finish welds and warped the receiver pretty bad. I practiced on some sheet coupons, but learned more when I started putting it together. Note to others, positioning and clamping the work properly helped me a lot over just having it laying on the table. I want to just start over on it to fix some other stuff that I could live with as it was coming along, but now that I have some experience, I can do it better. I put it away for a few months due to frustration, but I think I can get started fresh again. Some stuff I want to improve would be to trim the receiver a bit better, I got carried away in some spots, and had to do some clearancing in others. It worked, but I can do better now. I didn't line up the rails to the trunnion real good, I had to tweak them a bit to get it to work. If I had paid at bit more attention putting them in, it would have been easier. I shimmed the shroud to my RCM barrel and tacked it up. That worked and stayed square. Again while finishing it up, I didn't do it in small enough beads while alternating around the shroud and pulled it out of alignment. It still works, but is harder to change the barrel than it was. Initially, I had the feed box set too low, I had to pull the tacks off that and clearance the receiver a bit to get it right. It was obvious once it got to where it needed to be.  I admit, a bit more attention to detail, and being more patient with the welds would have saved me some problems. These were dumb mistakes, but worth mentioning to hopefully help someone else from making them. |
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Originally Posted By Pro_Patria_431: My biggest issue was my lack of welding skills. I got it tacked up, and it worked, but I got carried away doing the finish welds and warped the receiver pretty bad. I practiced on some sheet coupons, but learned more when I started putting it together. Note to others, positioning and clamping the work properly helped me a lot over just having it laying on the table. I want to just start over on it to fix some other stuff that I could live with as it was coming along, but now that I have some experience, I can do it better. I put it away for a few months due to frustration, but I think I can get started fresh again. Some stuff I want to improve would be to trim the receiver a bit better, I got carried away in some spots, and had to do some clearancing in others. It worked, but I can do better now. I didn't line up the rails to the trunnion real good, I had to tweak them a bit to get it to work. If I had paid at bit more attention putting them in, it would have been easier. I shimmed the shroud to my RCM barrel and tacked it up. That worked and stayed square. Again while finishing it up, I didn't do it in small enough beads while alternating around the shroud and pulled it out of alignment. It still works, but is harder to change the barrel than it was. Initially, I had the feed box set too low, I had to pull the tacks off that and clearance the receiver a bit to get it right. It was obvious once it got to where it needed to be. I admit, a bit more attention to detail, and being more patient with the welds would have saved me some problems. These were dumb mistakes, but worth mentioning to hopefully help someone else from making them. Originally Posted By Pro_Patria_431: Originally Posted By cms81586: What issues are you having? I was about to put together a post with some tips and tricks I wish I knew during my build. My biggest issue was my lack of welding skills. I got it tacked up, and it worked, but I got carried away doing the finish welds and warped the receiver pretty bad. I practiced on some sheet coupons, but learned more when I started putting it together. Note to others, positioning and clamping the work properly helped me a lot over just having it laying on the table. I want to just start over on it to fix some other stuff that I could live with as it was coming along, but now that I have some experience, I can do it better. I put it away for a few months due to frustration, but I think I can get started fresh again. Some stuff I want to improve would be to trim the receiver a bit better, I got carried away in some spots, and had to do some clearancing in others. It worked, but I can do better now. I didn't line up the rails to the trunnion real good, I had to tweak them a bit to get it to work. If I had paid at bit more attention putting them in, it would have been easier. I shimmed the shroud to my RCM barrel and tacked it up. That worked and stayed square. Again while finishing it up, I didn't do it in small enough beads while alternating around the shroud and pulled it out of alignment. It still works, but is harder to change the barrel than it was. Initially, I had the feed box set too low, I had to pull the tacks off that and clearance the receiver a bit to get it right. It was obvious once it got to where it needed to be. I admit, a bit more attention to detail, and being more patient with the welds would have saved me some problems. These were dumb mistakes, but worth mentioning to hopefully help someone else from making them. Hey, I have built a LOT of guns... From home builds to massive production shops. These 21s are not an easy build. I know we all strive for perfect, but honestly if you make one of these things run, it's an accomplishment. These are not an easy build! |
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Originally Posted By Pro_Patria_431: My biggest issue was my lack of welding skills. I got it tacked up, and it worked, but I got carried away doing the finish welds and warped the receiver pretty bad. I practiced on some sheet coupons, but learned more when I started putting it together. Note to others, positioning and clamping the work properly helped me a lot over just having it laying on the table. I want to just start over on it to fix some other stuff that I could live with as it was coming along, but now that I have some experience, I can do it better. I put it away for a few months due to frustration, but I think I can get started fresh again. Some stuff I want to improve would be to trim the receiver a bit better, I got carried away in some spots, and had to do some clearancing in others. It worked, but I can do better now. I didn't line up the rails to the trunnion real good, I had to tweak them a bit to get it to work. If I had paid at bit more attention putting them in, it would have been easier. I shimmed the shroud to my RCM barrel and tacked it up. That worked and stayed square. Again while finishing it up, I didn't do it in small enough beads while alternating around the shroud and pulled it out of alignment. It still works, but is harder to change the barrel than it was. Initially, I had the feed box set too low, I had to pull the tacks off that and clearance the receiver a bit to get it right. It was obvious once it got to where it needed to be. I admit, a bit more attention to detail, and being more patient with the welds would have saved me some problems. These were dumb mistakes, but worth mentioning to hopefully help someone else from making them. Some stuff that I learned in hopes of helping others…sounds like you’ve figured out a few of these already via exploratory learning. Reach out to me if you want to BS about getting it going. https://www.ar15.com/forums/Armory/Considerations-when-building-an-HK21-/9-558664/ |
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Originally Posted By cms81586: Some stuff that I learned in hopes of helping others…sounds like you’ve figured out a few of these already via exploratory learning. Reach out to me if you want to BS about getting it going. https://www.ar15.com/forums/Armory/Considerations-when-building-an-HK21-/9-558664/ #7: I had the same problem, and considered the same method. I even thought about a Pyrex cup. I ended up having a bushing turned and the shroud relieved to accept it. I had it made oversized and ground to fit. This was actually one thing that worked out pretty well for me. Attached File |
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Originally Posted By Pro_Patria_431: #7: I had the same problem, and considered the same method. I even thought about a Pyrex cup. I ended up having a bushing turned and the shroud relieved to accept it. I had it made oversized and ground to fit. This was actually one thing that worked out pretty well for me. https://www.ar15.com/media/mediaFiles/358054/20250729_221823_jpg-3596935.JPG That’s a slick idea. That weld was the most challenging of the entire build. |
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Originally Posted By TZLVredmist: Hey, I have built a LOT of guns... From home builds to massive production shops. These 21s are not an easy build. I know we all strive for perfect, but honestly if you make one of these things run, it's an accomplishment. These are not an easy build! Thanks. It was intimidating to start. Im actually kind of glad I messed it up at the end to remedy the "good enough" stuff I did. I got two kits anticipating problems, not knowing if I could salvage the learning one. I have another flat bent and welded, I need to order some new rails before I try again. I appreciate the encouragement guys. 
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Originally Posted By XJ01-08: Anyone do a build with a 14.5 barrel? Centerfire has em instock and looks like there's a HK417 flashider that's long enough to do a pin and weld. This is the one you want if it’s not the one you’re looking at. https://hkpartspro.com/hk-rotex-flash-hider Attached File |
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Originally Posted By cms81586: This is the one you want if it’s not the one you’re looking at. https://hkpartspro.com/hk-rotex-flash-hider https://www.ar15.com/media/mediaFiles/2980/IMG_1413_jpeg-3597551.JPG That's the one, figured I could get the mount for that and 14.5 would make a good suppressor barrel and get a 17.7 whenever they come back in stock |
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Attached File Attached File Attached File Centerfire shipped quick and I got my kit yesterday. Unwrapppping it I thought I really lucked out when I saw the shroud looked like it wasn't even used, then I saw the hammer and bolt carrier. Think I should try and get these swapped out? |
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Originally Posted By XJ01-08: https://www.ar15.com/media/mediaFiles/152238/2594_jpg-3600027.JPGhttps://www.ar15.com/media/mediaFiles/152238/2592_jpg-3600029.JPGhttps://www.ar15.com/media/mediaFiles/152238/2593_jpg-3600033.JPG Centerfire shipped quick and I got my kit yesterday. Unwrapppping it I thought I really lucked out when I saw the shroud looked like it wasn't even used, then I saw the hammer and bolt carrier. Think I should try and get these swapped out? I would. That carrier is trashed. The hammer is whatever but I’m sure they’d swap your trigger pack too. |
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Nothing to see here. Just making sector sketches… Actually just test fitting new barrels. Lighting was too good to not snag a few photos. Attached File Attached File Attached File |
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Originally Posted By 7insert: Finally got the low profile and light weight bag adapters I designed in. 7075, 2.8oz, mounts to existing holes with pins + eclips https://i.imgur.com/orX18VH.jpeg https://i.imgur.com/pOPQmwp.jpeg Like that setup. My box is an early style without the holes… |
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This thing works surprisingly well. Mag insertion is a little awkward. Attached File |
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Originally Posted By 7insert: Finally got the low profile and light weight bag adapters I designed in. 7075, 2.8oz, mounts to existing holes with pins + eclips https://i.imgur.com/orX18VH.jpeg https://i.imgur.com/pOPQmwp.jpeg Very nice |
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Originally Posted By bulletboy5150: Got a kit that should be here soon, which reinforcement rails do yall recommend? Did this version 21 ever have the flat faced rails or was it only the rounded ones? I am leaning towards the round ones if that is the "correct" ones for this era kit. Teardrop. Take some G3 stock rails and flip them upside down and modify as necessary. I believe some folks are making both versions as well. |
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Got my kit in today which is rated as very good and while the general condition of it is good as in not worn out there seem to be some issues. The feed box seems to have been demilled a bit too much with almost no metal left on rear of the ejection side, the rear of the shroud demill is crooked, and the front sight post is crooked (seems it was built that way?). Are these issues to be concerned about? The crooked sight post is definitely a bummer. Would Centerfire swap parts or an entire kit? Attached File Attached File Attached File |
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Originally Posted By bulletboy5150: Got my kit in today which is rated as very good and while the general condition of it is good as in not worn out there seem to be some issues. The feed box seems to have been demilled a bit too much with almost no metal left on rear of the ejection side, the rear of the shroud demill is crooked, and the front sight post is crooked (seems it was built that way?). Are these issues to be concerned about? The crooked sight post is definitely a bummer. Would Centerfire swap parts or an entire kit? https://www.ar15.com/media/mediaFiles/361801/IMG_1452_jpg-3604971.JPG https://www.ar15.com/media/mediaFiles/361801/IMG_1459_jpg-3604972.JPG https://www.ar15.com/media/mediaFiles/361801/IMG_1463_jpg-3604973.JPG I’d return that just based on the shroud being crooked. The feed box isn’t an issue and you’ll need to turn the shroud on a lathe anyways to clean it up anyways. Send it back and get another. |
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Originally Posted By cms81586: Correct. These 21’s were -6 to -8 right off the factory floor. Mine is still running about -6. My rollers measure 7.94mm with the bolt gap right down the middle of the acceptable range. I sent you a message about a build guide I made. |
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Was having some significant function issues with the 21 build. Ended up trying a different feed mech and she runs great. Tested the other feed mech and the mag adaptor works with a post sample sear pack and it runs amazing… ROF is up there with the 17” barrel. Much slower with the 14.5” barrel…much closer to an M240. RCM 36* locking piece installed. 17” barrel:  November 1, 2025 Mag adaptor:  November 1, 2025 14.5” barrel:  November 1, 2025 |
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Parrabellum Combat RT Parrabellum is working on a frt that uses a standard trigger pack and grip assembly, sadly seems like they're having issues as a 3 pos so might only be 2 but I think I could live with that. |
Joined:
Mar 2024
Posts:
650
EE: 100% (71)
TN, USA
First time on links instead of belts. She ran great! And if anyone tells you a 21 is hard to shoot or uncontrollable…tell them they’re going it wrong.  March 12, 2026  January 6, 2026 |
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Originally Posted By GBRUZK: Jumping in on the HK21 game and need some shared wisdom. 1. are HK21A1 barrels compatible with the FMP kit? 2. What are high wear parts that I should stock up on while inventory is available to keep this running? Yes Spare feed mech(s) Spare Bolt Carrier |
I really need to take it easy. Had a neck fusion done 3 weeks ago but couldn’t pass up the opportunity to put a couple rounds through the 21. Mainly brought it for some friends to enjoy…but temptation calls. Any guesses on the rate of fire? May 7, 2026 |
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Originally Posted By cms81586: Nothing to see here. Just making sector sketches… Actually just test fitting new barrels. Lighting was too good to not snag a few photos. https://www.ar15.com/media/mediaFiles/2980/IMG_1525_jpeg-3600551.JPG https://www.ar15.com/media/mediaFiles/2980/IMG_1537_jpeg-3600552.JPG https://www.ar15.com/media/mediaFiles/2980/IMG_1527_jpeg-3600554.JPG No kidding on the lighting! |
| Sorry to bump a dead thread, and apologies if the answer is in here somewhere already. I'm looking at buying one of these parts kits before they disappear. I don't know how to weld so a self-build is out of the question for me. Before I drop the coin on a kit, does anyone out there offer build services for these? |
RIP Sparky 1/15/15
"Did you not take into account I'm a noble savage? Unapologetic lifer for rock 'n roll!"
"Did you not take into account I'm a noble savage? Unapologetic lifer for rock 'n roll!"
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I believe this thread is not so much DEAD - as everyone here bought up kits and is either in line to have it/them built by someone, or is saving up to get in line to have them built - as these are NOT an average priced build project. ;) |
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Originally Posted By barbcue: After seeing his builds talking to him for over a year and having some parts fixed by him that i thought couldnt be that came back PERFECT. Im going with Jared at https://www.linkedkinetics.com/ He does absolutely fantastic work. Perfect attention to detail. He’s built multiple guns for me and has done some one off custom work. |
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Originally Posted By barbcue: After seeing his builds talking to him for over a year and having some parts fixed by him that i thought couldnt be that came back PERFECT. Im going with Jared at https://www.linkedkinetics.com/ Love it. I’ll reach out. |
RIP Sparky 1/15/15
"Did you not take into account I'm a noble savage? Unapologetic lifer for rock 'n roll!"
"Did you not take into account I'm a noble savage? Unapologetic lifer for rock 'n roll!"
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Jared at Linked Kinetics/Desert Yankee did mine; he’s backlogged but moved mine up a month or two due to the Virginia ban. It looks and functions great. Attached File |
If you bought an HK-21 kit get in here! (Now a build thread…) (Page 16 of 17)
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