I'm a big fan of most of H&K's stuff, so I'll send you to a few sites.
<a href="http://www.unsuave.com/~hkpro/">HK Pro</a> has lots of good info, including the GMG (Grenade Machine Gun), which fires 330 40mm grenades per minute:
Now THAT would be fun!
Also, go to <a href="http://www.remtek.com/arms/hk/index.htm">REMTEK</a> for lots of good info on HK, as well as other, small arms.