What it looks like you did here was open the add-ons list, then forgot about it and decided to go play on A.R.C.'s Army TDM, left after a while, checked for a better server, gave up, and left. You didn't start a server, meaning if we want to find the problem, we have to go find all of your addons, install them, and run them all at once to see if there is a conflict.
For now, do this: Start a minigame on your server, spawn a horse(Horse. Not Jeep. Not Ball. Horse.) then try to gun it down. Tell us if you can or cannot.