"The "rotary" feed mechs used on the HK GmbH factory-made HK23 and HK21 MG's are somewhat sensitive to the "pitch" of the various links (the distance between the centerline of the links). In the 7.62x51mm HK21E the gun was supposed to fire from DM1 non-disintegrating and German DM6/US M13 links and usually did but the variations could and did cause problems overseas in ammo made with used or poor quality or reused links where they streched slightly. Thus is the reason the last HK21E's were named "HK21E/M13's" because they were optimized for US M13 linked ammo and as such would no longer function reliably with DM1 belts.
The best advice is to try the linked ammo/links in the particular gun as there were slight variances in HK23 feed mechs and sprocket wheels over the years.
The M13 marked were to be used with the M13 US link.
The carrier was marked but that part didn't really matter.
The sprocket and the bolthead are what matter.
HK made 3 sprockets and 3 BH's.
What setup you have determines what link you can run.
I made the MM21E run both by using a combo of the 3 styles.
All 21"E"'s are the same minus the above mentioned parts.
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