A youth twenty is where we start them. Literally the recoil difference between a 20 vs 410 is so absolutely minimal that there is no point. And the odds of them hitting the clay go up dramatically. Then the cost element and availability of ammo make the twenty the winner by a large margin. We start them by locking thrower down and shooting straight out targets from center station. Once they gain some confidence we start moving them back to 16 yard line and letting the thrower throw. You have to build up their confidence first. Then they start overthinking it, then they discover girls and the whole thing unravels! Hah! I coach up to seniors in HS.
