YOu are wasting your time trying to use 20 gauge conicals in your .62 smoothie. They are no more accurate than a round ball in most guns. The problem is that most of the slugs, to save weight, are hollow based, and when the front heavy slug slows in the air, it acts like a badmitton birdie coming down over the net. It wobbles back and forth, and can be easily moved by any gust of wind.The Hollow base saves your shoulder, but at the cost of losing axial stability for the projectile. Because you have no rifling to rotate the slug, it does not maintain its flight on the same line. Now if you use a solid slug, either a very short one to save weight and your shoulder, or a long one that will definitely loosen your teeth, You don't have the problem of projectile design costing you axial stability: you just have a slug designed to be fired out of a rifled barrel to make it spin, and stay on a single line of trajectory. Absent the spin, it becomes unpredictable after it slows down to the speed of sound(1100 fps).
Stick with the round ball in the 20. Measure your bore with calipers. Then order the correct sized ball and patch them. Track has .595 and .600. There are some .605" balls out there, and you can order special molds for other sizes.
The " Secret " to round ball accuracy in a smoothbore is to keep the speed at or below the speed of sound leaving the barrel. A .600" diameter ball weighs in at about 235 grains, which is 3/4 oz. Inside 75 yards, it will penetrate both sides of an Elk, and any deer you might find to shoot. I know shooters who have killed deer out to 125 yards with a 20 ga. round ball, but they will admit they thanked their lucky stars, and years of practice for having made that shot. Most of their deer fall inside 50 yards, just like the majority of White tails taken every year.
Try using a 2 3/4 dram load( 75 grains of FFg) with a overpowder wad, then your PRB. The thickness of the patching you use will be determined by your bore diameter, so don't GUESS! measure it with calipers. My barrel if oversized, and I have to use 19 gauge wads to seal it. Then try shooting it from the bench with the barrel bare, with only lube on your patch, and alternately, by running a lubed cleaning patch down the barrel after you seat the ball. My tests indicate I get a little extra velocity lubing the barrel, but that standard deviation in velocity is measurably improved( less). That means the ball is leaving the barrel at close to the same velocity with each shot, improving the consistency of the gun's performance. All that is left is for me to practice enough to be able to consistently point the gun at the target acurately each time I fire. A rear sight of any kind helps me do this better.
