Borderlands ammunition SDU upgrade provides the same total ammo as my current total?
Solution 1:
There are 6 levels of SDUs for every ammo type in Borderlands. Whichever level SDUs are available will be shown, but you will be unable to actually purchase an SDU that does not exceed the one you already have.
Also, ammo SDUs do not stack, and purchasing the highest level SDU will have the same effect as purchasing all of them, for a given ammo type.