Surely the Mk III was scrapped after its dismal trial performance in outer space! When you have to use farm machinery to tackle the Dark Judges you know there are design issues at a level even Samsung would get a laugh out of.
That was one thing I disliked about the Dark Justice story (which wasn't bad as a whole). The moment they offered Dredd an upgraded gun, I thought "that's going to jam, isn't it?" And it did. I really like John Wagner's writing, but I'm sure a lot of us saw that coming a mile off. I understand why he did it. The mark 1 and mark 2 lawgivers can take out the Dark Judges' physical forms pretty easily with the incendiary and Hi Ex rounds, so Mr. Wagner wanted to put Dredd into a vulnerable position when facing the DJs. I just wish he had come up with something else. I'm not the brightest person in the world and even I predicted the outcome.
Since then I've wondered if they fixed the mark 3 and put it into circulation or if it's still being tweaked behind the scenes, or did they just scrap it? Some artists do draw a very different flat top design of lawgiver, but they did that before the introduction of the Mark 3 in Dark Justice.
Anyway, concerning (very small) continuity conundrums:
A prototype Mark 4 lawgiver appeared in the strip before the Mark 3. (Do you remember the story where Dredd got a whole new uniform with built in AI that went crazy? I forget the name of the story, but it starred Gok Wan. Sort of.) I guess their tech division is way ahead in the weapon's technology. Maybe they have another team working on the next model while another is still sorting out the previous prototype. Or that's how long each prototype takes to get passed for mass production.
I suspect the real-world reason is that the writer lost track of the current lawgiver model. Heh. If that's the case, there have been two mark 3 lawgiver designs! Or its the same one brought back again a year or so apart. They look different, but then so does the standard lawgiver depending on the artist.