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Well, I'm by no means an expert, but generally, I've found that the for the MP5 and HK33/53/93 series, the stocks will interchange (with the caveat you match up the buffer on the stock with the buffer or lack thereof on the carrier for the 5.56 variant). HK91/G3 stocks have 2 pins, rather than the single pin of the HK93 and MP5 guns, so they are different. Plus, the guide rod for the HK91/G3 series is attached to the stock, where as in the HK93 and MP5 series, it is not captured and just sits within the bolt carrier.Has there ever been anything written on parts interchangeability between the different sized rifles? 9mm, 5.56, 7.62. I assume if the length is right on the fore ends they will interchange. What about the stocks, bolt carrier groups, trigger packs, cocking tubes and supports and so on. I know newbie question. A lot of you guys probably know this stuff already but it may also help others.
The trigger packs are fully interchangeable across more or less then entire family and the MP5/HK93 trigger housings can be used across platform, though the HK91 specific housings will only fit the larger rifle caliber guns. The ejector levers will need to be changed depending on caliber (an easy swap once the pack it out) and the hammer springs are different (though a lot of people, myself included, just run the full 7.62x51 hammer spring in their pack regardless of which caliber they are firing).
As to carrier groups, I don't really see how they could be interchangeable, given the different calibers (thus different bolts) and lengths of carriers. Some of the parts from them might be interchangeable, but I'm not sure.
For the rest of the furniture, like forearms, I supposed they could be made to fit if the other caliber weapons had a similar stock length, though I'm sure some work would be needed (but people have done it, cutting down a wood HK91 stock to fit an MP5, for example).