I've always felt like the game doesn't spawn enough ammo as loot. Now in the beta it feels even less since I think ammo boxes and medical boxes don't spawn at the moment. I figured the best balance for ammo spawns would be if you didn't collect enough to sustain your current main weapon, but you do collect enough to stockpile other ammo types to the point you can rotate your main weapon without buying ammo every time you play. I developed and tested modded loot tables which achieves this, tested in singleplayer and coop.
Here are my results for random ammo amount
Notes: Shotgun ammo is unchanged from vanilla. Tracer ammo boxes were converted to normal. 9x19 and 9x18 amounts were too favorable probably due to their ammo type being the most likely spawn in the loot table. Tested with loot amount set to high. 9x39 and 7.62x54R accumulated very slowly likely due to being the least likely to spawn, but still felt balanced.
Weapon Box:
9x18: 12 - 30
9x19: 12 - 30
4.45x39: 12 - 30
5.56x45: 12 - 30
7.62x39: 12 - 30
9x39: 12 - 30
7.62x54R: 12 - 30
Ammo Box:
9x18: 10 - 24
9x19: 10 - 24
4.45x39: 10 - 24
5.56x45: 10 - 24
7.62x39: 10 - 24
9x39: 10 - 24
7.62x54R: 10 - 24
Random Loot:
9x18: 4 - 18
9x19: 4 - 18
4.45x39: 4 - 18
5.56x45: 4 - 18
7.62x39: 4 - 18
9x39: 4 - 18
7.62x54R: 4 - 18
If the loot tables would be adjusted in game, they would most probably be less favorable than this depending on the developers intention for ammo gathering, but the vanilla values just seem too low.