I have a anti material rifle, but atm am not high enough in guns to use it to its best potential. I am thinking if giving it to boone, but dont have much ammo. Do I need to give a constant supply ?

Yes, anything but their magical default gun/weapon needs ammo put in their inventory.

Yes, all companions only have magic limitless ammo for their initial weapon. Any weapon changes (including ones that use the same ammo type) require you to give them an ammo supply to be able to use it.

Damn, well there goes the awesme idea of boone with an AM rifle. maybe when I have more caps.

Dead Money has a very reliable source of .308 ammo though. (After completion)

I don't find regular .308 hard to stock in vanilla NV...between gunrunners and that guy in Camp McCarran they always seem to have 2-3 thousand, every few days. Unless you're still very low level, of course.

Except the AMR doesn't use .308 ammo.... it uses .50 cal ammo.