the wet dream is a transport crashing and a bunch of mk19's falling out the back, skidding across the highway rubbing their serial numbers off, then bouncing up to land gently in the back of your pickup truck, followed by a half dozen or so crates of mk19 snacks.
Not a real thing that I know of but what about a rigid"belt" that's 4 to 5 rounds long for when you're on the move, then you could switch out for a normal belt feed option when you are stationary