The recoil decreases as the mass of the craft increases, and since this kind of space rail gun can be extremely heavy (I had a 200 ton version, but it got lost when 2.1 came out) the recoil is not as much as you might think, and you can always balance this out by doing a small burn afterwards with the main engines. as for armor piercing, I wouldn't know as I suspect my computer just isn't powerful enough to calculate the collision of two objects moving relatively at more then 400 m/s so the bullets just glitch through the target. I've tested this many times on my computer and this always happened, however on my friends gaming mega tower It seems to work, although I didn't have the time to properly test it. It'd be interesting to see if someone else could find a way of solving this issue.