Rail gun is good on longstrike due to the tank ace ability helps increase mortal wound chance. Ion cannon is better on normal hammerheads because more shots= less variance and more damage consistently.
Longstrike is good with SMS due to tank ace ace ability helping it damage vechiles. Gundrones can combo with fireblade cadre, as long as it is attached you can enjoy the extra 4 shots at the higher BS value of the tank. Longstrike can shoot through hard to hit abilities as well because he buffs himself with fire caste exemplar ability to allow him to shoot at +2 bs(or to shoot at +2 bs if he does a normal move)
I did some mathhammering. To see some stats check out post 119http://www.advancedtautactica.com/viewtopic.php?f=52&t=26389&p=338794&hilit=Longstrike#p338794