I was able to get some information about the Dredd lawgivers directly from the armourer who made them. I will quote him here (with just some slight paraphrasing for order and clarity):
The Lawgivers for Dredd 3D were made by overlaying custom prop parts onto a Glock 17.
Several versions of the lawgiver prop were used in the film:
There were standard semi-auto blank-firing versions, Glock 17's with sears for full-auto firing, airsoft semi-auto blowback versions, a remote controlled silencer extending version, versions with electronics for light up displays, and solid rubber guns for stunt and background.
The front mag originally was meant to have a mechanism that would rotate out the (fake) specialized ammo, but this was ultimately not used in the film. The front mags used in the film were empty and the silver rounds that you see in the front mag are actually just 2D printed pictures of the specialist bullet heads and the mechanism that was built (but not used).
The lawgivers were spray painted a matte/satin black with silver weathering.