What, you mean like a revolver? Point the gun up, turn the revolver thingy with your right thumb, grab a bullet (or the whole series of bullets) with your left hand, bring the bullet(s)to the gun while you snap the thing open with your right hand, put the bullet(s) in, then close it and point at your target.
Simple as that. A pro should have no trouble with it. Note: Style used is for right-handers.
If you wanna save animation frames, you can ignore the part where you point the thing up. I just do that to keep the bullets from slipping out.
Disclaimer: Any sarcasm in my posts will not be mentioned as that would ruin the purpose. It is assumed that the reader is intelligent enough to tell the difference between what is sarcasm and what is not.