You need to buy a UMP lower and stock block. You also need to buy the 922r compliant parts. There will need to be some light milling work that needs to be done on the USC upper to accept the UMP mags. The barrels are fairly easy to remove. If you are handy with a hammer and punch this can be done in a few minutes. There are video's of this on you tube. A good place to find parts to do the conversion is HK Parts.net.