I have an AR at I can't seem to get scope reticles to go low enough on. I have had a recommendation to shim the scope. Has anyone done this, and if so how did it work? what is the best material to use for this? Copper, aluminum etc.? I have seen shim sets on Brownells and other sites for 10 bucks or so, but I have small metal around that I can use. is there an advantage to using a manufactured one?