As I look back, - - -
The original question was,
Shouldn't a .30-06 have a 24" barrel for optimum velocity?
I believe only the rifled portion of the barrel, without regard to extensions, brakes, or comp chambers, has any effect on velocity.
Original barrel for the 1906 cartridge was just short of 24 inches, and this was the standard by which the '06 was measured for many decades, if not always. Ned Crosman, in his fine old work,
Book of the Springield, rails against anyone who would tamper with this ordained-from-on-high standard. Claims it destroys the rifle's balance, ruins the ballistic integrity, and probably causes crop failures in Kansas.
Any time someone quotes the old, OLD, velocities, such as 2960 fps for somebody's 150 gr. load, the ammo company started out with a 24 inch, if not longer, barrel. As a matter of plain fact, though, if you go through all the arms company catalogs, you'll find that the majority of .30'06 rifles come in 22 inch trim. Some, indeed, are offered in 24 inch length. I believe friend and moderator
Art Eatman has the only 26-inch '06 of which I am aware. He hunts in wide open spaces, mostly, and makes good use of all that barrel. He obtains actual velocities near equal to some of the old .300 H&H loads, when the latter are fired from the later model, truncated barrel rifles.
I've always been quite satisfied with the velocities from my Ruger 77 and Remington 742 rifles with 22 inch barrels. One should really test and sight-in and practice with each individual rifle anyhow, using the actual ammunition which will be used in the field. Printed ballistic tables are very interesting and useful for comparisons, but mostly for comparisons with other ballistic tables.
Only two comparisons are worthwhile, in my thinking - -
1. Two similar '06 rifles with different barrel lengths, OR
2. Two similar rifles, one in '06 and the other in another caliber, with both having the same barrel length.
You pays yer money and takes yer choice . . . .
Best,
Johnny