This just happened to me. It was quite strange.
I got a 5 kill streak with hardline and had sentry gun on. So I threw down the smoke, and waited. As the helicopter hovered over the drop spot however, someone on the other team shot it down.
When the "sentry gun" landed, I saw it had somehow became ammo.
I guess the guy in the helicopter didn't put in the sentry gun before the helicopter got blown up...
Unfortunately the host migrated before then so the whole game didn't get recorded, or else I'd post the clip for you.
Anyways, cue "cool story bro", etc. Just thought it was a funny glitch.