When hanging the posters Matt pulls his gun and uses the bottom of the grip to hammer the nails. No one with knowledge of firearms would ever do this. The hammer spring is enclosed in the grip. Hammering nails with it could damage the action.
When hanging the posters, Everytime he switched the gun from his right to his left. It is very unlikely that a right handed man would drive a nail with his left hand. As stated before, much less with his pistol.