I recently had the chance to hit the range with a VP-70Z, the semiauto civilian version of H&K's 1970 machine pistol. It is notable both for being one of the few production machine pistols around (and it would only fire automatically when its optional buttstock was attached), but also for being the world's first production polymer-framed handgun. On a less positive note, it's next-most-known feature is it's trigger pull, which is very reminiscent of a staple gun.

The VP-70 was a very simple design, using a plain straight blowback action and an 18-round double-feed magazine. One of its other notable features is very deep rifle, which I had read was intended to reduce pressure by allowing gas to blow past the bullet. That sounded like a pretty goofy idea, but lo and behold, it's true. I was able to examine a couple recovered bullets, and the all show that the rifling groove area makes no contact with the bullet, and blackened scorch marks are present as well, a bit like cartridge cases from fluted chambers. In addition, a friend I spoke to has done some chronograph testing with a VP-70, and found that it produces significantly lower velocity with a given cartridge than other pistols with the same barrel length.

As for the trigger, it's long and heavy (kinda like everything else HK was making at the time...), but not necessarily as terrible as some folks would suggest. You have to approach it like a DA revolver, and get used to staging the trigger most of the way back and then pulling the final short distance when you have a proper sight picture. I found it interesting that Tim Mullin (with several thousand rounds' experience on the VP-70) commented in his book on SMGs and machine pistols that he would choose the VP-70 over an MP5PDW.