I have mentioned this before but I felt it was better to make a separate topic about this subject entirely so it is easier to find.
It is a well known fact that there is something wrong with custom ammo 6-10 as they were never truly finished. Well, as it turns out, the only thing that was never finished was the maximum ammo, which was copied from 1-5. The game still keeps the count without problems, and they work well down to model, ammo box quantity and so on.
It works like this:
custom ammo 6 = same max as custom ammo 1
custom ammo 7 = same max as custom ammo 2
custom ammo 8 = same max as custom ammo 3
custom ammo 9 = same max as custom ammo 4
custom ammo 10 = same max as custom ammo 5
As simple as that. This way, you can let 1 and 6 be grenades and 2 and 7 assault rifle ammo, as you probably leave the cap around the same for those types.
If this is too short for a true tutorial I apologize Mods, but I felt this needed to be known!