15 shots only for that 3 tons of ammo, and that being the only weapon. I am a bit worry about this. Also, please put ammo in the legs. If your arms go boom the resulting explosion would hurt the side torso and maybe your engine depending on how things are going. If your legs go out you are dead anyways.
CT and Legs are the first things to go on the Jenner. If they open my legs I don't need to make it even easier for them to kill me. Your argument favors putting ammo in arms not legs. If I put in my arms and get crit I lose the arm and get ST damage. If I get crit in the leg I lose the leg and I'm dead.
4t of ammo feels short with 4x SRM4s, can't imagine 3t with this one. About ammo placement, I went for side torsos on the oxide. Arms would not have been a bad idea if ammo were punctured evenly, but with the current system it might leave you out of ammo even faster if they cut your right arm.