First off ballistic data needs to be taken with a salt as your rifle may or may not achieve even standard loads numbers much less the mag or heavy loads. It gives you an idea but it kind of like pin the tail on the donkey as the only way to know for sure is shoot them from your gun, in your enviroment over a chronograph.

How Hornady and Federal achieve this is simply by changing the components used, what changes I don't know for sure but being a reloader I'd say the propellant is different then the others and possibly the primer. I have heard for years they achieve these by duplex loading.Aduplex load is mixing 2 different powders in a ratio ofa faster burning powder in a small quanitity known as the booster and the majority in a slower burning power. Duplex loads are considered a no no in the bottleneck reloading world so if any truth to these rumors of the major manufacturer's using duplex loads we'll never hear how they do it. The other rumor is spec powder not available to us common folk. Either way no one has let the cat of the bag on whats used inside. I haven't myself but some have taken them apart for a little CSI work, of that I've heard any conclusive results saying its this or that!

In regards to duplicating the numbers of these magnums offering or loads, a reloader should be able to beclose or better in the numbers department but has the ability to tailor other portions to therifle for achieving both performance on the chronograph and on the target. I admit to being not really up on factory ammo numbers and even less on the heavies but I recall a 139 Hornady in a 7mm rem mag "mag load" giving a MV of 3250fps. I not only achieve this in my own reloads I eclipse it as well as shoot sub MOA.

I wouldn't buy a cartridge based on the possibility of getting the few mag or heavy loads to shoot, look at the averages and thats what you should expect. If you want 300 MV buy one