The ideal Field Gun is the one's I've ripped off all my other units because they suck & now I have a pile of them sitting around.
IE.... AC5 & UAC5
Though really, they do make fine guns for infantry platoons.
At 24/27 tons respectively they can be pulled around by a "standard" 28 man platoon & take a hit or 2 from range w/o immediately loosing a gun.
And shooting 3 guns from 18-21 hexes tends to not attract the immediate "KILL IT" attention that will happen when the canon, 6x LAC5, platoon opens up from 15 hexes.
Sure, you'll get 1 nice round off w/ those guys, maybe 2, before something is close enough to SRM, Inferno, MG/Flamer, etc etc & wipe out the whole platoon.