Below is the modification to the pmag, this can also be completed if using a metal style mag (just makes the mat'l removal little harder). This will allow the bolt catch to contact with the mag follower. Also allows the mag to fully seat and lock into the mag well.
Below is the image of the bolt catch contact area that actually meets the follower after the last round, thus pushing the bolt catch pin up and locking the action open. Like I mentioned, on one of my VZ's, I had to slightly angle the portion that contacts the mag follower out a bit as it was just catching the follower but not enough to push the bolt catch up on the last round. The other VZ was fine and required no adjustment at the bolt catch.
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Below is how it should work when completed, my 2 VZ's work great and always lock open after last round. If you can install the mag well you can do this tweak. I took out the bolt catch and used a screw driver to adjust it forward.Work it slowly as not to snap it, and check the distance required to make good contact with the follower (may take a couple times to get it right).
Sorry about the crappy images, cell sucks for taking photos.
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Craig