RE: Need help with 12 ga slugs!
Theory tells me that from the moment that the slug leaves the barrel it is dropping towards the ground - because of gravity.
For the gun to be dead on at 50 and 1 inch high at 100 also tells me that you have issues between the scope, scope mounts and the parrelism between the scope / scope mounts and the barrel.
He still didn't say what kind of gun he used or the gauge.
If it is a 12 ga, I would switch to a 20 gauge because the weight of the slug / compared to the amount of powder charge in the shell is better than in the 12 ga.
AS for Sabot slugs, sometimes they might be as good as a regular old pumpkin ball and sometimes they are not worth a hoot. All that I am saying is that you cannot make a rifle out of a shotgun.
I thought about it all day and came to the conclusion that he would be better off with a centerline muzzle loader rifle if the season permitted it instead of the shotgun. Some areas of the country allows both - due to the fact that the bullet in a 54 cal muzzle loader is big and doesn't travel even halfas fast as a centerfire rifle.
I wasn't trying to be hard on him, I just wanted him to realize that you don't always have to go out and buy nothing but the best to try to make up for inexperience. Experience is the best teacher and practice makes perfect.