At a high level there are two types of “HK21”s belt feds out there.
The original/older version is the “HK21”. This is the gun with feed mechanism housing that wraps around the G3 receiver and is welded in place. The star drive feed mech itself then slides into the welded on feed mech housing and locks into place.
The original HK21s are in 7.62x51 only (although TSC did make a small number of 5.56 conversions) . To load these guns the belt is pulled through the feed mech using a loading tab so your access to the belt feedway is very limited so clearing jams, etc. can be more problematic. The HK21s only advance the belt as the bolt carrier closes. The original HK21s were only are designed to run off non- disintegrating DM1 belts vs. links.
The original HK21s also have a shorter shroud compared to the more modern guns. The original HK21s have a bit of a reputation of being not the most pleasant shooters in the world with excessive recoil.
Original 21s were built by many of the older names who are no longer in business Fleming, Vollmer, as well as gunsmiths who are still active today, Michaels Machine, TSC, RDTS, Terry Dyer, and there are plenty of home built guns over the years using imported FMP parts kits.
The other variant is the updated HK “E” series Hk21E and Hk23E. (there is also an HK21A1 and 23A1 in between but these are pretty rare)
The E guns are distinct from the older HK21 guns in that the feed mech swings down from the receiver. This makes the loading operation different in that you swing the feed mech down, load the belt and then snap the feed mech back up to the gun. (you can also load it via starting tabs by pulling a belt through the feed mech as well) . The E guns have the longer shroud (in factory spec form) in comparison to the original 21s. (Although there are non-HK-factory “K” variants of the E guns with short shrouds and correspondingly reduced length barrels)
The E guns can shoot either 7.62x51 or 5.56x45 via a caliber conversion kits that consist of a caliber specific barrel, bolt carrier group, and feed mechanism. They can also go from belt fed to mag fed with caliber specific mag fed conversion kits.
The E gun advances the belt with both the recoil and closure stroke of the belt in 7.62x51 increasing its belt pull capability over the older version (the 5.56 E kit only advances the belt on closure), the E guns have an upgraded buffer system, upgraded clubfoot stock, different barrel latch safety mechanism, and are designed around a 4 position 3rd burst fire control group.
The 7.62 E guns can also run belts or links and there are different generation feed mech sprockets and bolt heads designed for different belts/links. The factory E guns have a completely different full auto sear in that the sear and sear trip are all one piece and the bolt carriers have a completely different style trip on the bottom to engage this different sear arrangement.
There are basically two flavors of E guns in the marketplace. All German built guns that were built off a HK91 receivers and use all German parts from a demilled German E gun and the American clones using US made parts built by either Michaels Machines or Beltfed Precision Products. (although there are some mixed US/German part guns out there as well)
In terms of cost/desirability:
At the bottom of the pack are the older original HK21s. Guns built by a reliable builder like RDTS on a German HK91 receiver and parts command more vs. somebodies home-brew creation using FMP or unknown origin components. These original HK21s exist in both registered machinegun receiver as well as semi auto form.
The semi auto versions of these guns usually run around $8 to $10K or so and the RR versions around $30K.
In the middle tier:
Original Hk21 style guns that have been upgraded with various E parts. These guns may have the E buffer system, E shroud, E Stock, been tuned to run links vs. belts, etc. but still retain the classic welded on feed mechanism housing of the original 21 design. These guns exist both in registered receiver form as well as semi-auto. Depending upon the upgrades these guns are usually in $10K to $15K range and the registered receiver versions around $30 to $35K.
US built clone HK21E/23E guns by MM or BPP. These guns have the swing down lowers, caliber conversion capability but are designed as semi autos and to work with a friction sear. As a result they have a different bolt carrier designed to work with a friction sear, also have a hybrid barrel latch safety system, and the 5.56 bolt heads use full round rollers vs. the half moon style rollers of a German 5.56 gun. The US Clone guns run around $15K (for one caliber) and the conversion kit is usually another $5 to $7K. So say $20K+ range for both 7.62x51 and 5.56x45 kits.
At the top of the cost heap are the German built E guns.
These exist in both registered receiver form as well as semi-auto. For the most part these guns have been built by Michaels Machine, TSC, RDTS, and Terry Dyer. There may be others out there (Gerard Miller who did work for MM) but most of the all German E guns I have ever seen for sale were made by one of those guys.
The semi-auto German E guns will be pretty close to a factory E gun but will have modified bolt carriers in order to work with a friction sear (transferable sear or box). These all German semi-auto E guns are $30K and up depending upon which caliber(s) kits they come with , accessories, condition, etc.
The registered machinegun receiver based guns that were built off a HK91/G3 RR and full German 21E/23E parts kit are going to be the closest you can get to a factory HK21E/23E. They will take all factory parts, use the factory designed E sear system. The all German Registered Machinegun Receiver E guns are $50K and up. Again depending upon the caliber, accessories and condition. The German caliber conversion kits are in excess of $10K if you can find one.
IMHO, if you want the best and most up to date factory correct “HK21” than an HK21E built by one of the aforementioned builders using a latest generation M13 E parts kit with both 5.56 and 7.62 caliber conversion kits but be prepared to spend north of $60K for one.
If you want to use a transferable sear or box, than a German gun built off a HK91 using all German parts would be next in line but it’s going to be double (or more) the cost of a US built clone from MM or BPP.
I personally have a Michaels Machine 23EK / 21EK US parts gun as well as an all German HK21E/23E built off a G3 registered receiver and an M13 E parts kit (also built by MM) and am extremely happy with both guns.
As other posters have mentioned the HK belt feds are not the most robust and tolerant guns in the world. The parts are extremely expensive and in the case of German parts extremely expensive and rare (a German 5.56 barrel can cost more than $5K by itself). The star drive belt feed system is not the most robust and isn’t super tolerant in terms of dirt, links of varying pitch, rounds of varying length, etc. in comparison to guns with standard top cover shuttle arrangement like the M240, M60, or MG42. Its also a sheetmetal gun built off a legacy assault weapons based frame vs. say a steel plate constructed gun like a M240 or 1919.
The E guns are also extremely complicated to the point that many (if not most) owners are unable to take them fully apart or do basic part replacements, especially in the burst fire control group and the feed mechanisms. I don’t know exactly how many individual parts comprise a complete 21E/23E system but it’s an enormous list of components and the number of people who can fully service these guns in the US (if you can’t do it yourself) is an extremely small list.
All that said, I love the 21E/23E system for what it is, its flexibility to convert calibers and from belt fed to mag fed, find the engineering and how they work pretty fascinating, and personally think they are one of the coolest looking belt feds out there.
Hope this helps and if you have any other specific questions I am happy to try and help answer them.
