Other mistake: Shooting two forty-fives at once is hard. Willis must think so too. One gun fires blanks and the other gun operates like a real pistol, even emptying out shells, but is not firing. This is very noticable in slow motion during his first standoff.
Other mistake: Upon entering the bar to rescue the bartender, Willis shoots his first victim clean through a window. In slow motion you can see the gun is never fired and yellow flash is edited in.