The RedNeckEngineer has blogged about his modern RDP-like rifle build. He opted to make a hammer fired mechanism instead of the original striker mechanism. The striker mechanism works fine in the original fully automatic open bolt RDP, but is unreliable in semi-automatic builds that have to fire from a closed bolt (BATFE rules). RedNeckEngineer used a H&K G3 lower and fire control group to provide a hammer mechanism.
The barrel was cut down to 14.5″ and then a flash hider welded on to increase the length and make it 16″ legal. He removed the G3 pistol grip and replaced it with an AR-15 grip and a mechanism to keep his finger off the trigger unless the rifle is push forwards, allowing for easy bump firing (this is not pictured in the above photo).
[ Many thanks to Commander Zero for the tip. ]