should be unlimited ammo stacks that automatically increase in weight and size when more is added... pointless to constrain unboxed ammo to single item slots when you can easily split and combine it anyway. All picked up ammo should automatically combine with like for like loose ammo already in inventory. If you only have single slots available in inventory then just fill to max in each slot letting you combine to a larger slot at your convenience.

No it shouldn't be unlimited it should just have a higher stack count & nothing automatically combines upon pickup unless it's going into a container so bullets should be no exception.