That might give you better ballistics but would probably come at a cost to reliability and complexity. You'd still need to accelerate the sled to at least the s

That might give you better ballistics but would probably come at a cost to reliability and complexity. You'd still need to accelerate the sled to at least the same velocities they're doing now for the slug. Then you'd need to stop the sled in a very short distance which would put forces on the components that aren't there now. Finally, you'd need to reliably return the sled to it's starting position for the next shot. In conventional firearms you can move the bolt backward using either a gas return tube or blowback but those aren't an option for this type of weapon.