If it's not "fixed", then there are two basic alternatives (in short, SOMEthing should be done). The first is to simply disallow bulk mods on anything that would cause it to go beyond the 255 max (which would include Minigun's capacity as well). The second is to prompt the player, letting them know that the maximum allowed bulking (at 255) will be reached by applying the mod. Overall, however, I think the max should be a more rounded out number, like 200 or 300...255 was probably placed without much thought (ie, 8-bits are enough for everything so far *done*).
Personally I'd cap it at 300 for the armor/boots and cap 200 on the Minigun (or any weapon that reaches an absurd ammo capacity...BFG B3 hits 220, when 200 is good enough).