Not sure if this is a bug or not.
On beta39, playing as Blufor, I had the Mk 48 Mod 0 and 5 mags. Takes up 10 slots in the inventory which is normal. However, i died after about 20 minutes and went to change my loadout to something different. I tried the M4A1, MP5, etc...no launcher of any sort, standard pistol and I was only able to get 5 mags...that should only take up 1 slot. It said the max amount of available mags was 5. Once I spawned however I was able to go to Arming and get the rest of my ammo...and once dead again I was able to use all 10 slots.