Essentially the borrowed item wouldn't be able to be dropped/traded. But that's not the point.
We don't want a borrow system as that would obviously affect sales. Why buy if you can borrow? It would promote the "sale" of guns, which isn't what we want either. For example, you can "sell" a gun to a player by letting him borrow it forever.
It'll also be used to scam. "Player A said he'll let me borrow his BAR if I gave him 100 shells, but he didn't let me borrow it!"
As you can see, it could be heavily exploited and we do not support it.