Rimfire....The load I use is a WW fireformed case, Federal 210 LR primer, 36.9 grains of Win 748 powder and the #28120 Nosler BT bullet. This is a maximum load out of the Sierra handgun manual, and is safe with no excessive pressure signs in my 12 inch T/C Hunter barrell. This load chronographs at 2376 average muzzle velocity and 1504 ft/lbs of energy. At 200 yards, it is still going 2041 fps with 1110 ft/lbs energy. At 250 yds, it still is going 1962 fps with 1026 ft/lbs of energy. This load is just barely a 250 yd load when you figure that to get good clean kills a bullet should be traveling at aprox 2000 fps for proper bullet expansion, and be maintaining 1000 ft/lbs of energy for deer/antelope size animals. I don't think there is a 7x30 Waters 300 yard load that will dependably kill an antelope at 300 yards without risking wounding, and, being from MT and having shot many, many antelope, a wounded antelope will take you many miles of tracking, and even then you will be at high risk of losing it in the sage brush. They are one tough animal when wounded. The above load, in which I have killed numerous deer and antelope, is at 300 yards only traveling 1884 fps with 946 ft/lbs of energy. I'm sure a lucky 300 yard shot would kill an antelope if hit right, but the chance of wounding is not worth the shot. And, like the above post advises, the wind during the fall antelope season in MT is ALWAYS blowing and gusting hard, which moves low velocity bullets a long ways from your aim point. You need a dead rest, and you need to find and then stalk your animal to as close as you can possibly get. A 200 yard shot at an antelope with a 13 1/2 inch top of shoulder to bottom of brisket hight is one hell of a long shot in wind that is ALWAYS blowing. So plan on crawling through the prickly pear cactus carefully, and get as close as you can. The above load is the best that I have worked up, and the Nosler 120BT really has worked well for me. My longest shot at deer has been aprox 150 yds with little wind, and antelope at aprox 200 yards on a day when I could shoot directly upwind with little worry of wind drift, using a variable scope at 6x. The deer dropped in his tracks and the antelope ran aprox 20 yards, both shot through the lungs I sight in this load dead on at 190 yards, which puts me 2.5 inch high at 100 yards, -0.66 at 200, and -5.25 at 250. I hold dead on up to 200 yards with this load. Good luck on your antelope hunt, just get as close as you can with a steady rest.
And regardless of what your guide says, you can stalk close to antelope, as I usually shoot them at from 50 to 150 yards, and don't take chancy, unhumane shots. My 2 cents worth.
Gary