Of which one?
At one time, there was a radical anti-gun group that advocated doing a number of things to force their agenda, including browsing gun shows and slipping live ammo into display guns while the vendor's attention was diverted. Don't know if it ever happened, but I'd say that the chances are that it did. We hear about gun show discharges, and I often wonder if at least some of them are the result of espionage. If they're willing to do something like that...
On the C4...
During the Vietnam era, there were reports of Navy Seals booby trapping AKs with C4 packed ammunition, and a warning was issued that any found AK not be fired with the ammunition that was in it.
At any rate...C4 and its ugly sister Semtex are set off by heat and shock...not electricity. So, even if it hasn't been done, it's still a possibility and not worth the risk for a few pennies worth of free ammo.