The problem with readability of the gun on the right image is the left arm being in the way. It leads the eye down to the feet since the arm is brighter than the gun and it attracts the most attention.
Just some things to think about:
If you're not partial to the one-gun-arm idea, instead of working with my edit, just give the guy two gun arms and simply mirror the sprite. Or let either arm animate a transformation from the gun back to a regular hand *after* it is fired (don't delay the ability to shoot for some aesthetic as trivial as this one however -- big mistake.)
To be honest with you, if I had to choose, the right facing sprite vs. the left, I'd choose the left everytime. No need to be unique on something like this -- just flip the sprite. People will care MUCH more that the x value for shooting is different than about the time you spent putting into drawing a completely separate left/right standing/shooting frame. Trust me, most people will *not* think that much into it. Games have been flipping sprites for years.
Find another way to be unique and compromise any perceived 'quality' on this one -- it's not a feature enough people will truly appreciate to justify its existence. Even my edit is suffering from the same x-axis issue, and I've tweaked it to the best natural looking state the arm can be in for the right frame (aside from turning the sprite completely around and showing his back when he's firing -- though you may still have the x axis issue *and* it may lead to other issues with your moveset such as problems with your walking animation(s).