[@vietmyke] 20mm does seem to be on the heavier side of general purpose, but it's just big enough to squeeze in AMS viability. need to crunch some numbers on head size to see what kind of space AWs have for guns.

GAU-8 is an absolute beast. Warthog besthog.  Though, if your Raptor Custom has something that big, where does it go when it converts to walker form? does it unclip and act like a handheld weapon? does it replace an arm like Airmech's Warthog?

One problem with just flat increases to round size is that the power increase isn't linear. 3 50mm rounds won't come close to a single 150mm. I'd say 100mm is about as big as any unmounted FAW weapon can ever hope to be. and round size would depend on a weapon's role.
spray'n'pray? 35mm.
controlled burst? 40-50mm.
marksman? 55-65mm.
anything over would edge into sniper/Anti-Vehicle territory based on space-combat against opposed fleets. This is all for slug-thrower rifles though.

Mobility is king. Tanks are common because they're efficient at carrying big loads and putting big slugs down range, but air superiority is more important. with a tank that walks upright and stands at 17m like the Scout, you go from around 5km view range to the horizon, all the way to 14.7km. 3x longer sight lines and high maneuverability on the ground makes you a bigger threat and less threatened because you can see them coming, and threaten them sooner. also maximum traverse is better on a mech than a tank.

Maybe AWs are equally durable, maybe more, can't really tell yet.