Adds a Cannon turret using tank shells to the game.
All credits to everyone using that spreasheet :
I think the original was : https://forums.factorio.com/viewtopic.php?t=14957 thanks !
Nerfed the turrets bonus damages with research ( no more doubling of the damages, it would render the railgun obsolete completely )
physical-projectile-damage-5 : from 90% to 20%
physical-projectile-damage-6 : from 130% to 20%
physical-projectile-damage-7++ : from 100% to 20%
