I want to get on a 100 yd range soon. I also want to try some 5.56mm rounds in it.
Ammo is very important in the Mini be it 223 or 5.56. Purchase a few or more different types of ammo and see what works best for you and your rifle.
If your used rifle has not already had the trigger worked, I would suggest you have it done. It will not make the rifle shoot better, but it can make the shooter shoot better.
Clean the bore with some Wipe-Out foaming bore cleaner. It will remove everything the previous owner left behind with little or no effort.
Once you get it set up for 100 yards, take a 20 round shot at this target and see how she performs.