Zeroed at 171 yards it will be flat to only about 200 yards "The .450 is about a 200 yard cartridge for Class 2 game and perhaps somewhat less for Class 3 game. The limiting factor is the short, fat, light for caliber bullet's poor sectional density. Take care to drive the bullet directly into the heart/lung area with a broadside shot, especially if hunting big animals like elk. Hornady offers their 250 grain FTX bullet.
The latter claims a muzzle velocity of 2200 fps and muzzle energy of 2687 ft. lbs. from a 20 inch barrel. At 100 yards the numbers are 1835 fps and 1868 ft. lbs., and at 200 yards the remaining velocity is 1515 fps and the remaining energy is 1274 ft. lbs. The 2016 MSRP for a box of 20 Hornady .450 Bushmaster cartridges is $42.45, so the .450 is definitely a cartridge worth reloading.
Zero the Hornady factory load at 171 yards (i.e. +3 inches at 100 yards) to take advantage of its maximum point blank range (+/- 3 inches) and the trajectory looks like this (sight height 1.5 inches): -1.5" at muzzle, +1.83" at 50 yards, +2.99" at 100 yards, +1.53" at 150 yards, -3.11" at 200 yards, -11.59" at 250 yards." chuckhawks.com/450_bushmaster.html

100 yards definitely. 20pnyards you’re still good. 300 yards I wouldn’t, unless you’re a good shot.

Jennifer, this cartridge, nick named the "Thumper" will have a maximum point blank range (a range that when sighted in) will hit within a few inches without having to adjust aim. Just zero for 200 yards to take advantage of the effective range and point-blank trajectory. You could hold dead on a deer at any distance inside the zero of 200 yards all the way out to 280 yards.