The Hornady \"Custom\" 100g BTSP factory loads ...
are the most accurate factory 243 loads that I have ever witnessed. Last year I shot a 700 and a Win Model 70 with them after the guns were bedded, etc and the rifles shot 1/2" groups with the factory fodder...Hornady did a very good job with their ammo. I pulled the bullets, load was 42.3g of H4350. I duplicated that load in my 700 with the bullet touching the lands and shot a clover leaf with the bullets touching at 100.
Now as far as distance you can kill with that load.
Several years ago, after shooting p. dogs on a ranch that covered 63 sections of land, I got to shoot my speed goats. I shot them with a custom 223 loaded with 55g Sierra SPT flat base loaded at 3250 fps. I killed 4 goats with one shot each, the closest was 250 and the furtherest was 550, all bang flops. On a goat, it is not how much gun you have, it is shot placement, within reason.
Over the years, I had been using less and less gun. Started off with a 7 Mag, 270, 25/06, 243, then 223. I will not shoot another antelope, killed more than my fair share. However, antelope makes the best jerkey that you will ever eat.
Your post was, "How far can I kill an antelope, factory ammo with a 243"? My answer is how far are you going to practice? If you live out West, I would sight the gun in for 300 yards, see how your gun groups at that range, the put targets out at 400, 500, and 600 yards. You will figure out for yourself just how far you can kill one.
There is a world of difference shooting between 500 & 600 yards because the 243 really starts taking a nose dive between 400 to 500 yards.
If the wind is howling at 25 mph, it will take some pretty good shooting to hit one at 400 yards.