If you've never shot a gun at 800 yards, I highly recommend doing LOTS of target practice (at non-living things) before taking a shot at a living thing, and risking maiming it. There is no such thing as instant gratification in the long range shooting world, despite what all the gun magazine articles will try to tell you. You have to work your way out to your distance goal. You have to learn the equipment, learn how to use it. Learn the basics of LR shooting. Build muscle memory. Start at 100, then go to 200, then once you're shooting consistent groups at that distance, move to 300, etc... And I don't mean all in the same day.
And in all honesty, once you get past about 500 yards, the wind really plays a factor. So, 800 is not a cake-walk.
You will need LOTS of practice that you don't want to spend the time or money on... My suggestion stick with 100-200 yard shots.