Per Era Report 3052, an autocannon is rated based on how many kilograms of ordinance it can throw downrange and cycle a new burst within a ten second time period(the standard Battletech turn). AC2's are rated at 20 Kilograms, 5's at 50, 10's at 100 and 20's at 200. Caliber is variable when compared, fundamentally to rate of fire.
as far as long tom and Sniper Artillery pieces, from what I understand the Thumper is pretty much equivalent to the M110 203mm Howitzer that is the standard for NATO members for heavy artillery. That would make Snipers and Long Toms "Sci Fi" Calibers
