I like the idea and your reasoning behind it. But in my opinion it makes things difficult especially when doing events/trials, etc... I like the idea of buying it in the shop. It makes it cost money. Unless maybe if you either raise the price of ammo, or if this does get implemented maybe having multiple places in the base, and outside the base to get ammo. This will allow getting ammo more efficient during events/trials, etc... Ex. Putting buyable ammo in CIT, CSR, CQC area. There are armories in Bravo and Alpha.
So I am -1 for now, unless there will be tons of places where ammo can be bought.