Infantry weapons other than field guns and field artillery are abstracted. Ergo, infantry SRMs don't do the same damage as BattleMech SRMs, and infantry rifles do damage to BattleMechs, because all the damage is averaged into a single vs. everything damage stat that works against both 'Mechs and other infantry platoons.
The rules for field guns are inanely simple:
The platoon must be motorized or mechanized
Only autocannons or gauss rifles may be used, but any kind may be used.
A platoon may have as many tons worth of field guns as it has troopers.
Each gun has one ton of ammo automatically at no tonnage
You cannot fire them while moving.
That's essentially it. Field guns can make for fantastic cheap defensive positions, because AC/10 field guns, for example, are just as effective as if they were on a 'Mech, but if the enemy wants to kill them, they have to bring appropriate anti-infantry weaponry or they'll have a hell of a time doing it.
If you want to kill the crap out of 'Mechs, forget SRMs. Plasma Rifles are the support weapon to use. They deal 1.58 damage a gun, and allow you to deal heat instead of damage using your platoon at your discretion. A platoon with eight plasma rifles and 20 auto-rifles can deliver up to 23 damage out to six hexes. That's no joke.