I just starting modding OpenXcom yesterday by messing around with the Ruleset. I realized how easy it is getting to mod this game. I also read about the Modular rulesets and the upcoming OpenXcom 1.0 so I'm thinking about adding lots of new items eventually.
Right now, I turned the Auto-Cannon into something more like a Minigun. It deals less damage, the HE ammo has a AoE of 3x3 and the Incendiary ammo has a AoE of 1x1. Sadly, I had to tune down their damage considerably to adjust their AoE to what I was trying to aim. It only fires in Burst Mode, uses less TUs and their Clip Size has been increased so it works more like a fully automatic weapon.