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Posted: 7/29/2025 11:36:41 PM EDT
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I’ve had a lot of help and helped a few folks build their HK21’s in the process of building mine (Build thread from last year…) and after. This isn’t a build thread, but more of an AAR for a few considerations to think about when building yours to help you not make the same mistakes I narrowly avoided or did make. 1) Try to save as much of the cocking tube liner as possible. The ORF tutorial doesn’t show this and I didn’t do it. It made post-welding cleanup work a bit more challenging. Second picture shows what it should look like. Attached File Attached File 2) Weld your reinforcing rails and trunnion before cutting the magwell. Doing so after the trunnion makes it more challenging, but it’s possible. At least do the reinforcing rails first. Once you cut the magwell off the receiver loses a lot of rigidity and it’s very easy to warp. Attached File Attached File Attached File |
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3) Use clamps to line up your rails before you weld the rear portion of the trunnion and front of the reinforcing rails. If they aren’t lined up your rollers will hang up here and you’ll have failures to go into battery…especially with the extra drag the feed mech puts on the system. Attached File Attached File 4) when tacking your feed box before welding, ensure both the bolt protrudes into the center of the feed tray and deep enough to strip rounds off the belt, as well as ensuring the feed mech cam pin rides deep enough in the carrier to not jump track, and also rides the cam path all the way to the end to ensure the star wheel completes its rotation. This is a fine dance to ensure all of these things are occurring and nothing is dragging. Don’t weld it up all the way unless you know it’s working perfectly. Attached File Attached File 5) Get a piece of copper bar stock and turn it to the internal cocking tube diameter or buy something ready made. This both acts as an heat sink and keeps the cocking tube path clear if you weren’t able to save the liner. Any weld or slag protruding here after you weld up the cocking tube will be a complete bitch to get out. Ask me how I know. Also make sure the shroud is perfectly straight. Check both bolt carrier freedom of movement and that the barrel is centered in the shroud exit. Attached File |
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6) Use a new RCM trunnion. My FMP trunnion fits some barrels tighter than others. It’s obviously a little worn. If I were to do another build tomorrow I’d spring for a new trunnion. Attached File 7) this area between the feed box and the shroud is a PITA to weld. I had to grind this out and start over. In case you didn’t notice I’m not a great welder. Took a minute but I figured out if I ran the tungsten out quite a bit and turned up the argon flow that you can get down into this area quite a bit easier. Attached File Some of this may seem like common sense but it’s stuff that took me a bit of trial and error to get right. Hope it helps folks! |
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#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. Attached File |
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