Well whenever you buy ammo from ammunation and you go over the limit a message appears saying "You can only carry 9000 ammo!". Well this is annoying as I want to fill up my ammo to the maximum I can carry.
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(http://i.imgur.com/Z5kNyyN.png?1)
For example if I have 5,000 MG ammo and I buy 10,000 ammo it will tell me "You can only carry 9000 ammo!", however with this suggestion I will only receive and pay for 4,000 ammo which will add up to be the maximum amount you can carry which is 9,000.
In short: my suggestion is if you buy more ammo than the limit, it will give you X amount of ammo to fill you up to the maximum capacity.