CarJunkieLS1, I agree; physics doesn’t lie. But there’s other physical properties at work.
If the barrel does not move in any way after the primer fires, burns the powder, gases push the bullet through the barrel and exits, then indeed velocity spread will cause vertical stringing of shots caused by drop differences at target range. But that doesn’t happen. Go through the following in order listed and hopefully you’ll understand.
Recently, Geoffery Kolbe (owner of Border Barrels) presented the following:
http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/tuning_a_barrel.htm
Over a hundred years ago, the Brits proved why their SMLE’s shooting the 303 with a big muzzle velocity spread shot so accurate at long range. Click on the "PDF" link in this one to read it:
https://archive.org/details/philtrans05900167
Varmint Al’s website has several pages showing how barrels whip and wiggle before the bullet exits along with ways to “tune” the barrel’s resonant and harmonic frequencies to let the bullet depart at the right angle to compensate for it’s velocity so it’ll strike the same place down range.
http://www.varmintal.com/a22lr.htm
http://www.varmintal.com/aeste.htm
http://www.varmintal.com/alite.htm
http://www.varmintal.com/amode.htm
http://www.varmintal.com/atune.htm
http://www.varmintal.com/apres.htm
And finally, here’s Kolbe’s site that lets you plug in your .308 Win barrel’s profile dimensions and some load data to see where the bore axis points upon bullet exit. It only covers the barrel whip in the vertical plane in a free recoiling rifle. It will be less in a hand held rifle and also have some horizontal whipping because the center of mass holding the barreled action will not always be in the vertical plane of the bore/recoil axis.
http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/barrel_vibrations.htm
This is also a good explanation as to why several people shooting the same rifle and ammo will have different zeros with them.
The Browning BOSS works on this principle. Smallbore shooters often use tuners on their barrels as do some short range benchrest folks. Longer barrels used in long range matches typically have enough whip by themselves that no tuner's needed.
A half grain of powder will change the vertical average point of impact, but the accuracy will stay the same if the load's in a good range of charge weights to start with.
Regarding a given bullet having good accuracy across all sorts of bore and groove dimensions, as long as it's diameter is larger than the groove diameter, it'll shoot just fine. I've shot 30 caliber bullets at .3082", .3084" .3087", .3088" and .3092" in barrels with groove diameters from .3070" to .3080" and bore diameters from .2980" to .3010" all with equal accuracy at any range.
When the Brits switched from their .303's to the 7.62 NATO round for their long range fullbore matches where they had to shoot arsenal ammo (no handloads allowed). some lots of their arsenal 7.62 NATO rounds' bullets were only .3070" diameter. They had to get barrels made with .3065" groove diameters to shoot them well; they did great with them. A couple of that size were used shooting Sierra's new 30 caliber 155-gr. Palma bullet at .3084" diameter into about 3/4 MOA at 1000 yards.