*sight* Use proper english.
Anyway, now i see what you want.
Anyway, FPSC has a really bad physiscs system, also, in order for the props have the physyscs act on them, they need some interection by the player or other object.
It doesn't matter how heavy the objects are, they need some force to to touch them, so the physics take action, by having a box bellow it, when that one is destoryed it won't make much.
But stick with what i said, FPSC is not ready for physiscs, and the little it has, really sucks (sorry for word).
I suggest you find another idea for the player to pick up that ammo :/ (FPSC still has lots of bugs and unfinished feactures, wich are being worked with the new updated engine).
Edit: Just wanted to say: Don't take my answer as rude, i'd just like to make note that FPSC isn't like HL2 or any late game, the engine IS limited, and since FPSC was made for easy "level" making, some feactures were cuted off so other could work, and stuff like that, and regarding to physics, well, a physics system isn't easy to make, and much less cheap, at least we go the basics of them, on a cheap easy game making program.
My love is seperated in 2 parts, 1 of them is 3ds Max 7, the other; FPSC.