You can't put a tail on an L7, the bolt would be balanced backwards at all times and never move. At least not a tail that was the same size as the back bore in an Ion, you could sleeve that down I suppose.
Yes, a spring will help it move somewhat faster. Some systems it helps, others it isn't overly effective in. Some of the Lucky bolts were sprung forward, but I've never tested one to see with and without the spring how they perform.
You don't want to change it in the cycle, you want it as close to the ball as possible to minimize the volume the gas can expand to before doing useful work, which is wasteful.