Here is some info for you, copied from the RA site, but the whole article can be found on the RA site, go to latest news at the bottom, and then how to aim a rail gun, download the format, and half way down is a bit about recoil,
I have copied and pasted some info below that may be of interest, this must be wriiten by Rob
Rubber power was a big influence. I found that if the gun was powered up, the recoil caused the gun to shoot left or right, not so much up or down as one would think as with a handgun. How much deflection and which way was relative to how tight I held the gun, how stiff my arm was, which hand I held the gun in and, the power of the rubbers. I am left-handed and the recoil deflection for me is to the right. If I hold the gun handle with my right hand, it deflects to the left. I put this down to the way the handle kicks back into the thumb and then rotates the wrist.
With a 1,3m gun with double 16mm rubbers, set up with a 7,5mm spear, rubbers a little shorter than normal, the left or right deflection is as much as 100mm at max range. My definition of max range is the range a spear will travel and just come out the other side of a good fish. I set up the target such that the spear tip will only go through it and protrude out the other side by about 300mm once stopped by the shooting line. (Single wrap) This 100mm deflection is with a normal grasp of the handle. It improves to about 50mm with a tight grip and stiff-arm but is 150 to 200mm with a slack grip. My personal game fish guns are 1,3m with normal double 16mm rubbers and a 7mm spear. With this set up the lateral deflection at max range is only about 30mm with a normal grip. This is nothing at max range on a good size fish.