what are these?

Boxes of surplus ammo.
Buy them, drop them, pick them up and bam, surplus ammo.

Weaker than regular ammo, and degrades your gun faster, but you can get lots for cheap, so they're good for things like the LMG and minigun if you don't want to waste your good ammo.

Above is correct, and If you go into your inventory, Misc. section i think (only did it once myself), and left or right click on it, it drops out of your inventiry then when you pick it up you get the ammo that was inside it i bleive. I only did it once, but i think thats the way it went.

except Energy ammo actually degrades gun less (bug??)

When you surplus ammo, .22lr plinking ammo, or several other types of bulk ammo it its supposed to automatically break down but most of the time it doesn't, if you buy the box and it doesn't break down just drop it and it will. Same goes for boxes of primers, powder, and casings if you buy them for reloading. Some ammunition is actually better for the gun when you buy the cheaper kind like .38 special since it is loaded to lower pressures than the .357, some energy ammunition is supposed to be loaded to lower power too so it does less damage but is also better on the gun.