it is almost entirely a closed system and has been constructed so well, that it hardly deviates the plane's course.

Read up on Newtons third law. It doesn't matter how SLOWLY the recoil forces are applied to the satellite. It only matters that they are. To accelerate a slug of non-zero mass to the kinds of speeds necessary for it to be any worse than a mortar shell requires a huge amount of force. This huge amount of force WILL be applied to the satellite, in the opposite direction of the slugs acceleration. (allowances must be made for adding rotational energy to the satellite, but that would also wreck it's ability to continue firing rapidly and accurately). Actually, now I mention the rotational thing, perhaps it is possible to position the railgun and recoil-reduction systems in such a way that most of the energy is applied to the satellite as rotational energy. This wouldn't necessarily mean a change in orbital position. Then, if the timings were absolutely perfect, the railgun could be repositioned with respect to the centre of mass to the opposite side and fired again. This would require incredible timing though, and the launch rail would be rotating so I'm not sure how feasible this is.

It is a lot to compensate for if the same cannon that was used on the 130 is to be used on a satellite

I didn't mention the AC-130 because of the 105mm howitzer mounted on it, that's a silly comparison. The 105mm howitzer damages it's target primarily through energy produced by chemical reactions. A railgun slug damages its' target through expenditure of kinetic energy.

The 105mm shell "cheats" and requires less energy from its' launch mechanism.

Any orbital railgun that is capable of mass destruction on the surface of the earth will have recoil ORDERS OF MAGNITUDE higher than a present-day 105mm cannon.

Dropping a 3kg railgun slug with no initial velocity in orbit will be lucky to take out a garden shed.