Have a look at this thread.
Has there ever, or is there now, or will there ever be a beta mag made for the HK91??
Now for an add-on question...does anybody see a great thing if you have a registered fleming sear and an HK91 and a beta mag = a cheap HK21. Unless somebody knows that a 91 won't hold up to that kind of full auto fire. Anybody care to speculate?
The 21 has a quick change barrel; the 91 does not.
In addition, I would be careful with a 91 barrel in that configuration. If you burn on out there aren't that many more factory ones around and the barrel import ban makes it even worse.