Ammunition stacks should be based on cartridge bulk/weight.
Current values have some flaws.

The following is based on weight, not bulk.
Bulk should be similar.
I took the 40rnd 9mm stack as the norm.

Type------------Stack---Rounds/KG-------New stack

.22LR-----------50------~ 300-----------~ 130
.380ACP---------40------~ 90------------~ 40
9mm-------------40------~ 90------------~ 40
5.45x39mm-------30------~ 90------------~ 40
5.56x45mm-------30------~ 90------------~ 40
.357Mag---------30------~ 70------------~ 30
7.62x39mm-------20------~ 60------------~ 30
.45ACP----------40------~ 50------------~ 20
.308------------40------~ 40------------~ 20
7.62x54mmR------20------~ 40------------~ 20
12GA 00 Buck----15------~ 20------------~ 10
12GA Slug-------15------~ 20------------~ 10

Stack= Current stack values.
Rounds/KG= Real life values of rounds per kilogram.
New stack= Stack value based off of Rounds/KG.

If nothing else I hope it can be helpful for future balancing.

*EDIT: Makeshift formatting and clarification.