How a HK VP70 Pistol Works

Wikipedia: The VP70 is a 9×19mm, 18-round, double action only, semi-automatic/three-round burst capable polymer frame pistol manufactured by German arms firm Heckler & Koch GmbH. VP stands for Volkspistole(literally "People's Pistol"), and the designation 70 was for the first year of production, 1970.

The VP70 combined a number of design features that were innovative, or at least very unusual for its time:

It was the first polymer-framed handgun, predating the Glock 17 by 12 years. At 820 g (28.9 oz) unloaded, the weapon is lighter than most metal framed pistols of the time.

It has a double-stack, double-feed magazine; double-feed magazines are uncommon for pistols even today. These magazines hold 18 rounds, a rather high capacity for its original production time.
   
As on the Mauser C96, the stock was designed to be used as a holster when not mounted. On the military version of the VP70 this combination includes a unique feature: when mounted, a selective-fire switch, located on the stock, allows switching the weapon to a three-round-burst mode, with a 2,200 rounds per minute cyclic rate of fire.
   
The VP70 uses a spring-loaded striker like a Glock, instead of a conventional firing pin. It is double-action only, so the trigger pull is relatively heavy.
In lieu of a blade front sight, the VP70 uses a polished ramp with a central notch in the middle to provide the illusion of a dark front post.
   
The barrel has very deeply cut rifling; this was done to purposely vent gas past the bullet, placing less gas pressure on the slide, a critical part of the VP70's direct blowback function. This also results in slightly reduced bullet velocity when compared to other pistols with similar, or even slightly shorter barrel length.