HK21EUG History


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HK21's and RDTS's Involvement in Pioneering the Development of the 21EUG
With the advent of RDTS developed Ideas and Upgrades being used by other Individuals we felt a history lesson in RDTS's evolution our HK21EUG(E Upgrades) was needed.

1989-Recut Cocking Notch
I have had the pleasure of shooting Dealer Sample 21's before and of course figured I needed to own one. Fred Vollmer of FJ Vollmer who I had been doing some work for in the way of MP5SD's and Suppressors had offered some HK21's done on HK registered receiver's for $2650. Kind of high priced at the time considering sears were $300-$400 but what the Hell he even threw in a spare barrel so I picked one up. I think Terry Dyer might have been working there by then, he may have built this 21.
First thing I did was add a HK21E Butt stock because it had a significantly improved buffer. The problem was because the buffer was almost 2" longer than stock I couldn't get the bolt back into the cocking notch to load it before it butted up against the buffer. So I welded up the old notch and machined a new one about a inch forward. Now the cocking handle would lock in and hold the bolt back. I have to believe that since this was fresh from Vollmers, plus I had not seen it done on the few HK21's that were out there that we were the first to engineer and machine this simple "E" Upgrade. If someone had done it previous please let me know and I'll post it here.
1989-3 Shot/Burst group
Not to bad an Upgrade, Just had to make sure to have proper clearance for larger front of ejector. Did you know there are 2 styles of HK21 ejectors? Clearance for one style will not be enough for the other and it will bind a break.
1990-HK21 Suppressor (Video Link)
Couldn't resist again! Our Suppressor will take a 50 round pull through it and smile! Made the HK21 sound like a 22 rifle. Click the link and you can see and hear what it was like back then. The clip is from RDTS's "Machinegun Video and Instructional #1" that we still sell as well as being carried by other vendors. Keep in mind the Video is Copyright 1991 and is still relevant today with many HK Upgrades like the HK21, MP5SD, HK51 etc.

1990-HK21 Mounted on M2 Tripod (Video Link)
Not to difficult but was not the most portable unit. Was very comfortable and stable to shoot from. I look back on this in comparison to our MG3/42 Tripod adapter for the HK21. The MG3/42 with its slick periscope sight and all the elevation and windage adjustment you could wish for is a dream come true for the HK21.

2000-HK21E Carry Handle Mounts for HK21EUG and Mag Fed HK's
RDTS HK21EUG Development Program gets serious! From here on its officially HK21EUG(E UpGrades) time. Thanks to parts becoming available again we went to work. This E upgrade enabled the HK21 user to carry his weapon comfortably without burning his fingers! Really an easy upgrade for the HK21 but until know was never done. The more difficult part was designing the HK21E mount to go on the Mag Fed HK's. To be fair we have to acknowledge that Dan Hoodenpyle had briefly sold some mounts of his design in 2001-2002. RDTS is currently tooling up to make a reproduction of the HK21E carry handle because it so hard to get one from HK Germany. We will include the 5mm pin, spring and e-clips in our production version along with mounts for the HK21 and Mag Fed Weapons.

20001-HK21EUG High Strength Receiver Box with cast in slots for RDTS HK21EUG 120rnd Belt Box
This unique part developed from a need. We had been accepting customers HK21 part sets and orders for complete builds for about a year with the expectation that a sheet metal receiver box would be available. Todd at SW and Murry Urbach both had this part on their to do list. After a year we decided to go ahead with our own. Frank DeSoma of POF-USA who I had met 10 years previous but lost touch with gave me a call and asked if we had any projects he could help us on. He had no idea of what we were trying to do. Now Frank is very proficient in Solid Works a CAD program plus being a former Tool & Die maker this was a God Send for us. A customer/friend volunteered to let us keep his Qualified Manufacturing HK21 pushpin Machinegun a little longer as it was in the shop for Refinishing. We used his Factory Perfect HK21 as a measurement model for our receiver box. Turns out it was the preferred one as it was a Portuguese model with the deeper box well. Everything came together and you now have a High Strength alternative to sheet metal, that still has the exact same look as a factory HK21! Plus we incorporated into our design slots to accommodate the Worlds First No Adapter HK21 Ammo Box for 60's era 21's!
Why we believe this is a better part: Click Here

2001-HK21EUG 120 rnd Belt Box (Worlds First No Adapter Belt Box for 60's Era HK21's
The following is to inform the public that we are the original designers and anything stated or advertised otherwise is fraudulent and of suspect business practices. We worked hard to bring you the best others should consider that. We rely on the industries sense of Integrity.
While Frank and I were brainstorming on the HK21EUG Receiver Box I suggested we go a step further and figure how to hang a HK21E Belt Box on our HK21EUG. Between Frank and I we came up with an ingenious design. It became apparent that the HK21E Belt Box would be too difficult to attach without major work or adapters on the HK21. So we came up with a 4 point attachment method incorporating 2 1/16" slots on each side of the bottom of the receiver box. We then designed a catch mechanism that uses one button press to detach the Belt Box from the HK21EUG. I never liked the wire assembly that HK uses on the HK21E Belt Box as it is very difficult to unlatch and almost impossible with gloves on. I remember some video tripods we had used in the past that had spring steel push buttons to index the legs when extended. So Frank whose knowledge of manufacturing and components found the ideal part and attachment method. It only takes a gentle squeeze with your thumb and index finger and its open to feed ammo or another click falls completely open for loading! What's really versatile is RDTS's Belt Box can be easily adapted to any HK21 with a little mill work. Keep in mind with the varying depth of different HK21 Receiver Boxes a little work on the rear of our Belt Box mounting plate might be necessary. Generally this is on Factory push pin HK21's with shallow Receiver Boxes. To date we have sold 100's of these boxes without one return or complaint!

2001-HK21E/PSG1/MSG90 Forward Assist designed for HK receivers
Again we got busy! We had a HK21E we were building for a client in the shop. Basically stripping a Dealer Sample and fitting the parts to a registered push pin HKG3 Machine gun receiver. I machined off the HK21E Forward Assist Housing then disassembled the Button Assembly. Got together with Frank DeSoma again and started measuring into the CAD program. Frank found an outfit to make a Laser SLA from the file and even added in shrinkage for the casting. We took the SLA's to a casting house and had Molds made for the buttons and housing. That done we took the CAD drawings of the internal Button components and had those machined up.
What makes this housing a little better?
If you've ever had a HK21E, PSG1 or MSG90 laying around the house you'll notice the side of the receiver where the Forward Assist goes is smooth. What I mean is if you look at a HK91/G3 receiver you'll see a hump running down the receiver side. What we did was in RDTS Forward Assist Housing we allowed for that with a depression in the bottom of the housing. Not a huge deal but it makes installation a bit easier. Also those who think HK21E's Forward Assist Housing is machined, it sure has that cast look to us.
Why does a HK21 need one?
Why does the best of the best HK21E have one? In a perfect world rounds would strip off a belt like butter, we don't live there! When you strip the first round by knocking the cocking handle off the cocking notch you don't have the full stroke that you would have when the weapon fully recoils on firing. Generally this is not a problem, but there are times that maybe you have a sticky link, dirty ammo or a variety of reasons that the round wont fully chamber. Your only alternative without a Forward Assist is to "double stroke" meaning pull back the cocking handle enough to snap the carrier and chamber the round. If you pull to far back you'll strip a second round and have a double feed. Kind of a mess! It is always better to have the Forward Assist and not need it than to need it and not have it. Hummm Where I heard that before?

2001-Site Tabs for Quick Detach Picattiny Rail Systems
This was a needed once you installed a Forward Assist. The Forward Assist kills off part of your HK Claw Mount attachment points. We have machined HK21E/MSG90 style site tabs for our customers that insisted on using HK21E/MSG90 Scope mount but those baby's are pricy, the cost of the tabs being machined and the mounts! We felt that a clean look with open sites and the ability to use a variety of Optics was something we needed to take to the drawing board. Again an relatively easy solution. We designed a set of Weaver Style tabs about an inch long. We relieved them on the bottom so they would site low on the receiver. They were spaced to accommodate a ARMS Quick Detach Throw Lever Picattiny Rail. Now you can have several Optic solutions ready to go with no re-zero problems. Plus with out Optics you have the clean HK21E look with the low profile tabs. When we setup the tabs we use a laser cartridge in the chamber and a laser on the mount aimed at a reference target in the shop so they are pretty close before we weld them on.

2001-Cast 21E Style Rails
The HK21 used rails that went over to top part of the rail channel whereas the HK21E has rails designed to go over both sides of the rail channel for a little more stability in the receiver. We measured and had a mold made to produce the HK21E style rails. We used these rails in a lot of our HK21EUG builds and PSG1/MSG90 projects. We have since changed to CNC machined rails that are factory perfect and lie within .001 of the rail channel! Plus the top of the rail is the same thickness as the Factory E rails. Ever look at some of the HK21E style rails out there? Something look different, its the thickness! Check out the pics of our HK21E rails and their fit. No comparison, look at the others with rail height twice that of factory.

2001-HK21EUG Produced for Sale

2002-HK21EUG Feed Mech Modification
This involves a little lesson in what feeds a HK21. We believe that the HK21(60's) was designed to use non-disintegrating belts similar to the MG3/42 belts. But with the readily available M-60 linked belts it figures that may be used in the HK21. Matter of fact the HK21E was offered in several configuration on what fed it. HK offered a feed mech for a long time that accommodated both types of belts, M-60 or MG3. Then they came out with a Feed Mech for M-60 liked Belts, and the Non-Disintegrating belts would jam in it. If you lay a M-60 linked belt next to a MG3 belt you'll notice that the M-60 belt spaces the rounds closer to each other than the MG3 belt. What happens when the M-60 belt goes thru the HK21(60's) Feed Mech as the Bolt recoils over the round ready to chamber it pushes it down on the spring loaded feed drum. When this happens the round behind it is forced into the star wheel that is feeding the belt. This smashes the case neck of the round on the feed drum, check it out next time. So this is why your told M-60 links require hotter ammo, because that round on the drum doesn't want to go down (it can't) so something has to give, plus a lot of drag on the carrier. We looked at the problem and came up with an easy solution that will not affect the reliability with Non-Disintegrating belts. You can truly have both and forget the need hot ammo stuff, what's up with that? We simply relieve the back side of the star wheel to take the pressure off! and since the rounds are fed by the front of the star wheel no harm is done!

2003-CNC HK21E/PSG1/MSG90 Rails

2003-Custom Bolt Head Rollers

2003-Tripod Adapters for the HK21EUG Series

2004-HK21K UBF Program Underway

2005-HK21K UBF Produced for Sale

2006-HK11K UMF Produced for Sale




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