I like it, but I've never been a fan of using a Gatling to represent the UACs. There are plenty of autocannons that have double the firing rate of guns like the Bushmaster, mainly those that use revolving chambers.
Yep I'd prefer to depict faster firing autocannons with a single, maybe double/triple barrel setup. This is how it's been set in recent canon though, and I'm expected to deliver art that resembles the reference.
The background and bullet effects are really well done! Bit of a personal preference though but I think the leg part might make more sense if it was bend down and the feet was larger - to support such a big unit.