Things like bedding the action, free floating the barrel, trueing and squaring the bolt and action, I would hope are done with everyone's rifle. If you don't, accuracy will never be as good as it could be. If these are done correctly, ammunition choice should be the only variable.
Standard Deviation in our ammunition comes from the ammunition not being exactly the same from cartridge to cartridge. These include powder charge, seating depth and temperature. If you shoot your cartridges through a chronograph, the chronograph should have a Standard Deviation readout. A good rule with ammunition is a SD of 1% of your velocity. So if your velocity is 3000 feet per second, you want your SD reading to be 30 or under. When that is achieved with your ammunition, your groups should be consistent if all else is equal. There are very few factory loads that meet this requirement.
Once that is completed, manipulating your bullets seating depth can then decrease your overall group size. Rifles are very indivual and they generally will shoot only a couple different loads at their very best.
Lastly, it is not a given a 1" group at 100 will equate to a 4" group at 400. Some rifles will do this, but aside from good custom ammunition, your barrel and squared action have to be very good quality. Average barrels won't maintain their groups as distance increases.
Bottom line, like most things, but the best you can afford, have a gunsmith give your rifle a tune up, then work up a custom load your rifle shoots well.
