Ok I'm going to get flamed to hell and back for this, but right now the most viable Forge Gun for AV and AI is the Assault Forge Gun with some situational exceptions.

AFG: at the advanced tier and proto the charge time is 3 sec with 1375 and 1500 damage respectively with no STD variant for 500 DPS at proto.

I'm going to say "let's not touch the Assault Forge Gun."

However...

The Assault Forge Gun retains splash, does higher DPS and better alpha than the standard Forge Gun. (500 DPS base at proto)

The current Standard Variant charge time is: 4 seconds. I don't recommend changing this.

Damage profile is 1200/1320/1440 this is the worst forge gun for anything but sniping infantry, where it beats the Breach hands down. it does not stand on it's own as an AV platform.

I propose the Standard Forge Gun damage be buffed to 1500/1620/1740 so it remains ahead of the Assault Forge in Alpha if not DPS. (DPS to 435 base at proto)

The Breach Forge Gun is the worst of the lot by far. Not only does it have a 6-second charge time, but it immobilizes you and has the WORST DPS in the AV crowd. I would suggest bringing the DPS to a whopping 420 from 350 at base proto by dropping the charge time to 5 seconds and reducing the movement speed penalty from 100% to 80%.

the Holy Jeebus alpha strike of 1750/1925/2100 doesn't seem to me something that should be buffed further.

TL;DR: Don't change the assault, it's the most balanced, make the standard hit a bit harder, make the breach hit a bit faster.

All numbers pre-skill to match up to the unmodified assault forge guns. Skill calculations will affect all of them, obviously. DPS calculations assume a four shot magazine and do not take into account reload times.