1-66 means you would need to have a 66 inch barrel for the projectile to make one complete revolution in the barrel. So this is a slow twist. Slow twist barrels shoot roundball. They normally do not shoot conicals very well. Some of them will shoot ball-et and some people claim they will shoot minie ball but I have yet to find that the case. Fast twist barrels like the 1-28 shoot sabots and conicals better.