Here are the schematics for each gun, MK II being the newer
http://www.2000adonline.com/?zone=dredd&page=tek
MK I have 2*12 round clips half of each clip is General Purpose, the other half is decided by the judge but can only be 1 type of round not mixed. So you end up with a clip say 6 rounds GP and 6 rounds Armour Piercing, then another 6 round GP and 6 rounds High Explosive. A Judge generally carries 2 clips in the gun and 3 spares. They also have 3 Heat Seeking, but these must be fitted externally to the gun.
MK II holds 30 rounds, but has a built in self-capping mechanism, so you have all ammo types available, and when you select each round it puts warhead type on the shell. So you can have any number and any mix of rounds up to the 30 shots, they generally carry 4 clips. This also has a short-range taser type thing built-in called a stun shot.
In both situations if they are near their bikes, they have access to a lot of spare ammo, also the bike carries the scattergun, basically a pump action shotgun.
The only other item to note is stumm gas, basically a cs gas hybrid, incapacitates.
I hope this helps, anymore and I would probably be infringing copyright of something.
Steve