+1 Big Duke. What you need is the 21 kit and a good 91 receiver unless you are building a beltfed 91. The only part of a 91 you need is sheetmetal of the receiver with the serial number on it for registration. If you have a factory HK91, you would be better off selling that gun and buying a HK21 already built or a good parts kit and receiver and wait for a couple of years to have yours built. I bought an actual new FMP parts kit in 2000 and new FMP receiver in white to build mine. There aren't many 21 parts kits left out there. If you have a bunch of cash you could buy a HK21E parts. They are $10K to $12K for a new sample that has been cut. I have found that many times when we try to make something different out of what we already have (be it a gun, a car, a girlfriend ...) we end up spending more money than we would have if we just sold what we had and bought what we wanted. Just my experience. If you have a registered receiver G3 (NFA), that's a whole 'nother deal.