Vaquero,
As others have said, if you are shooting out to 1000 yds or more you want a .45 cal., 1-18 twist, square bottom groove barrel. Barrels, bullets, moulds, wads etc. are all readily available. Also,
it would be nice if the gun weighed in the range of around 12 pounds, otherwise recoil can be a little much with bullets in the 530 - 560 gr. range. If your rifle is that heavy it my be difficult to lug it around all day. I have a 12lb. rifle that shoots a 550gr. bullet with 80 grs. of Swiss and recoil is not light. A mercury recoil suppressor in the butt stock makes it tolerable.
Good luck,
Kevin