how'd you get the paddle release on your HK?
There are a few companies that will do it. I think Arizona Gun works is one, it came to mind right away. Otherwise if you're careful you can do it yourself, but if you do it wrong you've made a machine gun by ATF thought process.
As for the STG .22 I've been itching to get one bad. I saw one at an ace hardware a month or so ago and quickly informed people to check it out. I was really impressed with it and the wooden locker it comes in is just plain cool... I'm jealous.