I'm coding a double barreled shotgun, and I don't want it to emit the shells until it reloads.

The way I'm doing this is making the rounds have no shells, and making the magazine have the sprite of two shotgun shells side by side. It is drawn behind the gun, so you cant see it until it is ejected out. It gibs two shotgun shells, to give the effect of two shells being ejected.

The problem is, how can I make the magazine gib whenever it hits the ground?

If you want it to be "near, but not yet on", then you need to use Lua to detect the ground pixels directly below the falling magazine, and if < X, gib. But probably the easiest way is to give it a negative RestThreshold, so that it gibs immediately.

Give it very weak properties in the .ini I guess, make the impulse damage extremely low.