Seems to me that any time there's more metal contacting the bullet, the greater [the] chance that more variables come into play. Besides, folks who shoot highpower rifles the most accurate[ly] have very little case neck tension on the bullet anyway.
It's really easier to have uniform case neck tension by having it light in the first place; neck length doesn't come into play when this is how it's done. And ammo that's been handloaded [which is] then let set for several weeks or months will have a greater release force needed with long necks because of dissimilar materials bonding between bullet jacket and case neck/fouling. There's more area to bond when longer necks are used.
All that aside, lets go back to when the .30-06 and .308 were the only cartridges allowed in NRA match rifle matches. Both cartridges were used in barrels of equal quality as well as the same action and stocks by several top shooters in the USA. Both cartridges were used in matches at ranges from 100 through 1000 yards. Many thousands of rounds were fired in both types. Bullets from 168 through 200 grains were used with several powder, case and primer combinations.
In comparing accuracy between the .308 and .30-06, folks who used each quickly agreed on one thing: .308s were two to three times more accurate than the .30-06. In the early 1960s, it was also observed that competitors with lower classifications using .308s were getting higher scores than higher classified folks using .30-06s; at all ranges. By the middle to late 1960s, all the top highpower shooters and virtually all the rest had switched to the .308. The Highpower Committee had received so many complaints of ties not being able to be broke between shooters using the .308 and shooting all their shots in the tie-breaking V-ring, something had to be done to resolve this issue. In 1966, the NRA cut in half the target scoring ring dimensions.
At the peak of the .30-06's use as a competition cartridge, the most accurate rifles using it would shoot groups at 200 yards of about 2 inches, at 300 of about 3 inches. The 600-yard groups were 6 to 7 inches and at 1000 yards about 16 inches. As the high-scoring ring in targets was 3 inches at 200 and 300 yards, 12 inches at 600 and 20 inches at 1000, the top scores fired would have 90+ percent of the shots inside this V-ring.
Along came the 7.62mm NATO and its commercial version; the .308 Winchester. In the best rifles, 200 yard groups were about 3/4ths inch, at 300 about 1-1/2 inch. At 600 yards, groups were about 2-1/2 inch and at 1000 about 7 to 8 inches. It was not very long before the .30-06 round no longer won matches nor set any records; all it's records were broken by the .308 by a considerable margin. Some accuracy tests at 600 yards with the .308 produced test groups in the 1 to 2 inch range. These were 20 to 40 shot groups. No .30-06 has ever come close to shooting that well.
At 1000 yards, where both the .30-06 and .308 were allowed in Palma matches, the .308 was the clear-cut most accurate of the two. If top shooters felt the .30-06 was a more accurate round, they would have used it - they didn't. In fact by the early 1970s, the scoring ring dimensions on the 800 - 1000 yard target were also cut in about half due to the accuracy of both the .308 Win. over the .30-06 and the .30-.338 over the .300 H&H when used in long range matches.
Most top highpower shooters feel the main reason the .308 is much more accurate than the .30-06 is its shorter, fatter case promotes more uniform and gentle push on the bullet due to a higher loading density (less air space) and a more easily uniformly ignitable powder charge.
Military arsenals who produced match and service ammo in both 7.62mm and 30 caliber have fired thousands of test rounds/groups with both. They also found out that with both ammo types, the smallest groups were with the 7.62 by about 50 to 60 percent. M1 rifles in 7.62 shot about twice as small of groups as .30 M1s at all ranges. When the M14 was first used, there were some .30-06 M1 rifles that would shoot more accurately. It took the service teams several years to perfect the methods of making M14s shoot well, but when they did, they shot as good as M1s in 7.62.
There will always be folks who claim the .30-06 is a more accurate cartridge. All I have to say to them is to properly test .308 vs. .30-06 and find out. Theory is nice to think about; facts determine the truth.