I found this thread already: /showthread.php?t=69814&highlight=shotgun However, it really doesn't quite solve what I need. If I were to put a ship into space and attached a 12 gauge shotgun to the back (so that the recoil went entirely into moving the ship) and then fired it, how much force would it produce to move the ship in the opposite direction? The equations are simple. I know the mass of the bullet (1 oz). I know the velocity when it leaves the shotgun (1610 feet per second). m = 1 oz v = 1610 ft/s a = vt F = ma I just need to know t. The problem is that I have no idea how long it takes for the bullet (a slug) to go from 0 up to the 1610 ft/s it is travelling when it leaves the barrel. One of the responders on the linked page simply assumed 0.1s. Is that a real value? Where would I find out the real value?