Yup, a tough one. I may have to settle with this one pose...

I had also noticed in the photo that he seems to be tossing it through the air, not quite in control.

I could always claim that it's the result of the freeze frame action pose!

Will have to think some about it.

How to visibly tighten his grip or make the gun look more stable...?

It is already a bit of a stretch, getting him and his weapon out of little mouse in a hurry...

 

Rotating his body, ducking below the door frame, lunging from the seat, getting the first leg out out and about to transfer his weight to it, getting that awkward gun past the steering wheel, pushing the door open at the spot of the open window with the other hand and extended arm, all while yelling at his buddy to get out, because a DAF Kittyhawk has spotted them and started to bank around!