The longest I ever shot was 500 yds using iron sights and prone. I seldom got out of the black, but it was not a big deal. Big difference between 500 and 1000 yds, Real shooting starts at 1000 yds.
I was a Marine, got drafted into teaching hand to hand because I had done some fighting and the Corps liked my shooting skills. But I was a pilot and my life depended on my flying skills not on hand to hand or shooting. I tried to avoid doing either of the latter. However I still have a fascination for the skill of long distance shooting. If the conditions are right I could see the track of the bullet down range.
A lot of guys during Vietnam got drafted into being a sniper because of their shooting skill. Many got no sniper training because of the pressures of the war. Now the young men are getting real training and shooting the Barret which is a popular rifle. I used to try to pick the brain of my young sniper friend. He had little to say about the calculations as his partner would feed the data into a computer and give him the solution. The hardest part of his training was the conditioning as you can't be breathing hard and shoot precisely.