RE: Lehman Great Plains Rifle.
The Lyman Great Plain Rifle has a 1-60 twist which is best suited for shooting roundball. The Lyman Great Plains Hunter on the other hand has a 1-32 twist which is best suited for conical bullets and could also shoot sabots.
To answer your question can this rifle shoot sabots? Sure it can. It just might not do a very good job. If you do shoot any sabots (which I would not recommend) they better be small and light weight. Your best best for that rifle is patched roundball. If it is a .50 caliber they normally shoot a .490 and some have better luck with a .495 ball and patch combination. You will have to experiment with your rifle to see which it likes best. If the rifle is a .54 caliber then respectivly it will shoot a .530 or .535 ball and patch combination.
Personally, make sure the rifle is very clean. Lyman likes to pack a lot of shipping grease in their rifles. The rifle comes from Italy and is made by Investarms Company. They are a very good quality shooting rifle. Also since you have a slow twist (I am guessing) you can push a roundball pretty hard. 90 - 100 grains of FFg powder (other then Triple Se7en cut that back 15%) and a patched roundball should give you excellent results.
You have a great rifle. They are good shooting, well made and although a little heavy, well balanced. They should shoot excellent groups for you out to 100 yards and even further with practice...
Good luck with your rifle. Anything else we can answer for you, just ask...