the 20ga. is perfect for deer and deer size game, heck I even use it on black bears. Your gun is fine as is, but shotguns are very iffy on the type of ammo you use, some may work better then other, you have to try different brands. I would stick with 3" slugs (make sure your gun can handle up to 3") for better performance I would invest in a rifled/cantilever barrel with a scope in the 3-9x40 range. With this your shooting will improve plus you will reach out further then you would with your current barrel