I'm the one that voted "insane". I would recommend NOT building one....
The HK21/11 series is based on a G3. Think of a G3 with reinforcement plates on the sides. OK. The roller-lock system is prone to wear by peening the inside of the receiver. This causes some slop and can cause issues. IIRC it was to do with lock-up and the inside of the "rails" peening out and causing cycle issues.
The quick-swap barrels are prohibitively expensive. Was as much as PKM barrels but more difficult to find. If Cal's didn't have it, you couldn't buy it.
On a positive note, the resale value WILL be maintained! These seem to be popular. Most likely due to the HK quick-change trigger pack that allows you to easily switch to class-3 if/when you get a registered sear.
I did a great deal of research on these a few years ago and arrived at the conclusion that while a clever design/package, it just wasn't up to par with its bigger cousins.
Do yourself a favor - spend some quality time with the search tool on HKPRO and (gasp) hk54.com (Todd's board). If you can still dig up those old threads, you'll read what I read about the reliability issues.
Arguably, it may be a non-issue if you keep the number of rounds you feed it down to a manageable rate.
I don't know - I'm on the fence. I do think it was mikesmachines or something like that that was making the HK23 (.223 version) around the time the Shrike was going to start shipping () but I stopped looking shortly thereafter.
Personally I think I'd tackle an RPD before a -21.