He means an all copper bullet, so being that you are limited to factory ammunition, you likely choice would be something loaded with a barnes bullet.
The 165 accubond or 165 scirroco is what I would go with of those choices, if you want to save meat then extreme velocity is not your friend, the high velocity of the lighter bullets will cause more blood shot and more meat loss. Also, hunting pronghorn here in Wyoming often times means shooting in high wind, so the heavier bullets will have a higher ballistic coefficient and cut the wind better even with their lower velocity, giving you slightly more room for error. However, if your shooting at that distance you will need good accuracy too, so what you end up hunting with may be dictated by what the rifle shoots best.
As far as ideal cartridge for a pronghorn/whitetail hunt, a 300 weatherby is not exactly what I would call ideal from what I know of your situation. Don't get me wrong, it's a good round, especially if elk is on the menu, but if your new to hunting you may also be new to shooting, and IF that is the case, the heavy recoil and expensive cost of the 300 weatherby is simply not a good choice to learn long range with. Even with a good muzzle brake taming the recoil (not the inefficient radial brake that comes on them) the percussion can cause a new shooter some problems. Something more along the lines of a 7mm-08, .260 remington, 6.5 creed, 6.5x284, or even up to a .264 win mag and possibly 7mm rem mag is better for a new shooter, though a light 7mm mag can have significant recoil as well. These rounds allow more shooting due to less shooter fatigue from recoil, more affordable shooting because of cheaper ammunition, give good trajectories, and still have enough power to hunt big game, especially if your intended targets are only white tail and pronghorn.
A 300 weatherby (while expensive) is certainly a good long range hunting cartridge, especially when handloaded with 200-230 grain bullets, but my concern would be more the ability of a new shooter to hold the necessary 5"-6" groups or less at the max 600 yard range you listed with a larger magnum cartridge in what is likely a relatively light rifle (sub 10 lbs). Pronghorn are not very big critters, so you need to be able to hold good groups at the range, because things certainly are not easier in the field.
I may be way off base, and you may be a very experienced long range shooter that has just never hunted (which is perfectly fine) but if that is not the case, and you are relatively new to shooting and long range, I would be cautious of getting too much gun, which will simply end up teaching you bad habits and poor form. All too often people new to the sport fall into this "You have to have a magnum to kill" mentality, when in reality you need an accurate shootable rifle and time behind your gun getting comfortable and accurate with it.
I don't intend to stand on a soap box, just want to make sure someone new to the sport is getting a good start!!
