In the OOBs that I'm planning now, I treat light mortars as heavy crew-served anti-personnel. Heavy mortars are light bombardment same as old-style infantry guns, and actual field artillery pieces are medium, heavy and long-range bombardment. I think it's important that we do not get too bogged in technical detail for each weapon/system, as long as there is sufficient variety, so that players are free to describe and RP their units and their weaponry in a way that fits their game. Many weapons are technically different and look very different, but perform the same task/job on the battlefield. Good example would be anti-tank weaponry, where a heavy AT mine, any sort of AT gun, an HEAT RPG and a ATGM look all very different and are radically different to build and operate, but in essence perform the same task - of defeating tanks.