Aaron, to your original question ("how can I improve my grouping with my .300?"). There are two parts to this question - you, and your rifle.
To find out how well *you* are shooting, and perhaps to improve it, you should find the most accurate, easiest-to-shoot rifle that you can. A known-good .223 or 6BR F-Class rifle, or a 6PPC benchrest rifle, fit the bill better than a .308 F-Class rifle, which fits the bill better than a .338 magnum target rifle, which fits the bill better than pretty much any factory rifle. With such a rifle (accurate and easy to shoot), go out and see how well you are presently able to consistently shoot. From this you will learn how good your aiming, trigger squeeze, mental control, wind reading etc all are, and you'll learn (for example) "I can always count on shooting 1.2 inch 5-shot groups at 200 yards" (or whatever). If you want to learn how to shoot better, use a super-accurate rifle to figure out how to do better. It is virtually impossible (as in, it takes *years*) to improve your shooting with a rifle that is less accurate than you are - don't waste your time this way, if you can possibly help it!
To find out how well *your* .300 mag is shooting, it needs to be fired by a shooter who is at least a good a shooter as the rifle is. That could be you (if you know how well you are able to shoot), or it could be another person. But whomever it is, it is important to know that a wide shot is the *rifle*'s fault and not the shooter's, otherwise you can't really pursue and fix the fault.