Here's a few ideas to make the rifle cannons more effective in the field.
Armor Piercing Ammo
This idea comes from the high explosive squash head round that flattens against armor upon impact and detonates, causing pieces of armor to spall off the inside to do damage to crew, components or spark fuel or ammo to explode. I'd say that actual damage is less than the regular ammo, but doesn't have the -3 damage modifier, perhaps doing 2/3rds actual damage since this is explosive ammo. The +1 to-hit modifier should be kept as well.
Light Rifle Damage: 2, -4 critical hit modifier
Medium Rifle Damage: 4, -3 critical hit modifier
Heavy Rifle Damage: 6, -2 critical hit modifier
Flechette Ammo
I'd say that this could be unchanged from the autocannon version, except that the -3 modifier should be taken off before halving damage against armored targets with BAR 8 or higher.
Precision Ammo
This could be possible by using fin-stabilization instead of gyrojets to increase accuracy. Maybe this could be a -1 modifier on the target movement modifier (minimum 0) instead of -2 for the autocannon version. Ammo is still halved per ton, and the -3 damage modifier against BAR 8 & higher armor still applies.
There we go. Some ideas to expand on the rifle cannons that give extra flexibility, but still leaves the autocannons as the better option.
Questions? Comments? Opinions?