Rule3, yes, there is there more variables with everything else besides if the brass is Lapua or Norma. (For everyone) That’s why the best .30-06 Garands put bullets down range in 2.0 to 3.0 MOA compared to 6mmBR benchrest rifles doing it in 0.2 to 0.3 MOA. They are different animals, too. One’s an equestrian carefully bred and built to do precise things on an obstacle course in good weather by especially trained and capable humans. The other’s a draft horse built to deliver armament across battlefields to do well in all sorts of weather by normal humans. One can be refined for best performance much easier than the other. One’s virtually untouched and not aimed by humans when it performs while the other is totally touched and aimed by humans. Here’s why.
The cartridge....
6mmBR powder charges are short and fat. That lets them ignite very uniformly from shot to shot. .30-06 ones are longer and skinnier. They don’t ignite as uniformly from shot to shot. Which one do you think will produce a greater muzzle velocity spread?
That 24 caliber round has a 40 degree shoulder going from .456" diameter down to .272" diameter. Compare that to the 30 caliber round’s 17.5 degree shoulder at .443" tapering down to .340". ‘06 cases have been measured for shoulder setback from firing pin impact with readings .004" to .007" depending on friction between case and chamber shoulder. BR cases so measured show about .001" shoulder setback. Which one do you think will enable a more uniform primer firing?
One’s .243" diameter bullets have a few pounds of force needed to push (or pull) the bullet out of their case neck. The other’s .308" diameter bullets have 20 to 40 pounds of force to do that. How much difference does that make with a 20% spread in each to their pressure curve shapes and muzzle velocity spreads?
The rifle....
Bolt action benchrest rifles have three moving parts; trigger, bolt and firing pin; all are easily put back into battery for each shot very repeatable from shot to shot and have minimal influence on how the barrel whips and vibrates from shot to shot. Garand service rifle’s moving parts include the bolt, hammer and firing pin, op rod, hand guards, magazine follower parts, rear sight parts and the barrel’s mid point fit to the stock’s ferrule through their lower barrel band. ‘Tain’t easy to make all of them go back to exactly the same place from shot to shot. Otherwise, their varying fit after each shot changes the way the barrel whips while the bullet’s going down it; the muzzle axis won’t point to the same place from shot to shot. Epoxying hand guards to their barrel helps but the rest have to be precision fit and matched for the best repeatability from shot to shot possible; maybe 2 or 3 people are still around that do that right.
Rifles held and aimed by shooters....
13-pound benchrest rifles are fired in free recoil untouched by humans except by a finger on a 2-ounce trigger. They’re resting on supports that stay in place for each shot and recoil back very repeatable from shot to shot. And go back into battery exactly the same for each shot. The shock of their trigger stopping at its limits moves the rifle very little, if at all. That 9 pound Garand is held by a wiggling human (their heart beat move muscles holding bones in position; they’re aimed somewhere inside a 3/4 MOA area at best slung up in prone) pulling a 5-pound trigger. When that trigger’s pull slams into its stops, the rifle moves a little. Nor is the rifle repeatably held against one’s shoulder nor supported the same by the off hand on its fore end. As the shooter’s position is not 100% repeatable from shot to shot, that impact further moves the muzzle axis off the desired point by the time the bullet leaves because the center of mass/resistance to recoil ain’t in the same place for every shot. Which one will shoot most accurately by humans if both test with equally accuracy when shot from a free recoiling accuracy cradle?
High magnification scope sights on benchrest rifles can be aimed at the same point on their target square repeatability within about 1/50th MOA. Ambient light direction and amount make no difference. Metallic sights on a Garand can be aimed repeatably on bullseye targets to about ½ MOA repeatability. Which is why the old “sight sayings” for M1's rear sights are: “Light’s up, sights up; light’s down sights down. Sun to the right, sight right, sun to the left, sight left.” All because of the way the front sight post is illuminated and seen/positioned relative to the target bullseye. Enough said about sights.
Regarding brass quaity....
Prepping and sorting brass makes a difference in accuracy of about 1/10th MOA in short range benchrest matches in the best ones with 1/3 MOA accuracy at 100 yards or 1/2 MOA at 300 (yes, that’s what the best have done shooting several 5- or 10-shot groups averaged for their aggregate group size). The best accuracy with .30-06 Garands tested in accuracy cradles with arsenal match ammo was about 1.5 MOA at 300 yards; good lots of commercial match ammo or hand loaded new cases was about 2/3 MOA. 7.62 NATO versions used by the USN and USAF teams were about 40% better. Which one’s easy to see the benefits of case sorting and prepping when normally fired?