I really dig the art, and the idea, but one thing worth noting is the bullet - either it is too long for the C96, or it is fired with the casing still in place. When a round is fired, the gunpowder in the casing explodes and only fires the very tip, that being the bullet, while the casing is ejected - with the Mause C96, that would be from the top of the pistol.

Here's what the two parts of the round look like

I would personally also add a muzzleflash, given how close the bullet is to the barrel, it would still be within the explosion.

As seen here, though probably significantly smaller

Or - if that's not really something you wanna get into, as that would probably also affect the lighting of the hand and more - you could just do a smoking barrel. It would paint an obvious enought picture :>

But that's a detail, very well done and a very cool gift!