I have built a couple of these "TTH" rifles: also known as a .22-6MM with a 8 or 9 inch twist barrel. The bullet to use seems to be a 75 grain Hornady A-Max. Hit 'em in the ribs and its lights out. I am in the process of building a .22-250 Ackley with a 9" twist for the same application. Not legal in all states (Colorado is one of them), but a pretty lethal tool for medium big game.