Roland starts out with a wounded right arm, using his left to shoot and to pay the doctor with a coin in the hospital, but on the bus he takes pills and drinks a Coke with his right arm while holding his left arm limp.
On the beginning of the final fight with the man in dark the gunslinger walks in with only half of the bullets on his belt filled. Once he looks at the boy and draws his guns the belt is full of bullets.
While Jake is being tied up in a chair, at first he is being tied up by Tirana. In the next shot he is being tied up by a man wearing a green jacket. The shot after that we see Tirana walking away, implying that she just finished tying up the boy.
At 1:17:41 - Walter hands off Jake to the women in red, as she places him onto the chair. Then the scene changes angels and a man in green can be seen locking Jake in and then it cuts back to the red women walking back.
In the end sequence a second bullet is fired to alter the trajectory of the first bullet mid-flight. In reality the same caliber bullets are shot at approximately the same muzzle velocity and it would be impossible for the 2nd bullet to catch up with the first.
Understanding that this is a fantasy story and that Roland's pistols are somewhat magic there is a bit of a glitch here. The pistols are based on the Remington 1858 army model and, as such, should be single action pistols meaning that the person firing them would have to cock the hammer manually each time they're fired. Roland operates these pistols as double action pistols (never manually cocks the hammer) throughout the movie firing them one handed.