Let me try to clear up the machinegun debate. It's a complicated topic after all.
Today, machine guns are typically weapons that come in
5.56mm - 13mm cals.
Guns with a
20mm+ cal. are usually called cannons.
Cannons that have a high rate of fire are refered to as autocannons.
So the 30mm gun on the A-10 can't be refered to as a machine gun, it's an autocannon.
Wikipedia gets it right as well:
A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rifle bullets in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute.
An autocannon is a rapid-fire projectile weapon firing a shell as opposed to the bullet fired by a machine gun. Autocannon often have a larger caliber than a machine gun (i.e., 20 mm or greater)
PS: Shells typically explode on impact while bullets don't, but there are exceptions.
PS2: A multibarrel gun can fall into either of those categories.
PS3: Yes, there is a grey area, between those two. But one thing I can say for sure, a 30mm gun can never be called a machine gun.
PS4: I use the MadCat a lot, it has machineguns. I haven't really found them useful for killing Atlases, or any other mech for that matter.