Knex Concept: Multi-Staged Bow.
Well, I don't know whether this has been done before.
a design for a multi stage, railgun style bow.
It uses as many bows as you want linked up.
Here is a picture and how it works.
1. The trigger is pulled, letting go of the first bow's elastics.
2. The elastics catch on the bullet, propeling it forward.
3.When the bullet reaches here, the trigger will set off the next bow, boosting the bullet even more out of the barrel, 4.
This will be complicated to time as it will all happen in a fraction of a second. The key to make this work is to time the second bow to fire just after the first, like the SwagBoss. Even then it would take some fine-tuning to get right. Build it if you want, I don't have parts for something so big.