I don't believe this to be true, but I can understand how a rumor like that would proliferate through the shooting world. It stands to reason that the 30-06 is a turn of the century cartridge (the last one, of course), so surely handloaders can do a lot more with modern bullets and powder than it was originally designed to do. Combine that with the receiver-failure history of the 1903 Springfield rifle, which had some forging issues in early runs, followed up by soldiers accidentally feeding it the wrong cartridge with a slightly over-sized bullet, and poor cartridge brass quality in those early days, and you get a rumor like this.

But unlike many of the "turn of the century" cartridges before it, the 30-06 was never designed around black powder, and frankly, it ran almost as hard on day 1 in 1906 as it does today, almost 110yrs later. The 1903 springfield was actually designed as a solution to the Krag rifle that was unable to sustain high pressure cartridges.

And of course, when someone hears that "cartridges have to be down-loaded in deference to the old SPRINGFIELD RIFLES," their brain can easily forget that the cartridge in question is the 45-70, and the "springfield rifle" in question is the Trapdoor, not the 1903. A simple case of mistaken identity here.

Then I suppose you can throw in the fact that there were 2 revisions made after the original M1906 cartridge was released, one of which was made to reduce the recoil!! The original M1906 bullet was a 150grn pill, that was eventually replaced by a 173grn bullet in the M1 cartridge (not to be confused with the rifle). After repeated soldier complaints, the 150grn bullet was brought back over a new powder, designated the M2. This reduction in bullet weight is sometimes argued to be an indicator that the 1903 rifles were weak-by-design, so some might confuse that this change was made because of the RIFLE's durability, but it was actually driven by SOLDIER durability instead.

So it's understandable that folks that never handled a 1903 Springfield or a 1917 Enfield and that don't understand the histoy of these rifles might confuse themselves about the 30-06's cartridge potential and its history.

In my personal experience with the two, a reloader can "speed up" a 30-06 a bit and almost reach 300win mag levels (about 100fps over factory), but it's not worth much, and equally, a reloader can "speed up" a 300win mag about the same, so the net gap between the two stays about the same.

One can argue that since the 30-06 is chambered in the same model with the same diameter dimensions for barrel and action as the 308winchester - which is the same diameter cartridge with a higher pressure standard, AND that the 30-06 is chambered in the same rifle dimensions as the 300win mag - which is a larger diameter cartridge, meaning a thinner sidewall as well as a higher pressure, that the 30-06 should be able to be operated at ~62-64kpsi safely. The idea here being that you're exerting the same resultant force on the action as what these other cartridges would, with the 308 action having the same hoop strength, and the 300win mag action having LESS hoop strength. Structurally, as an engineer, I can't argue with the logic, other than to point out that, as a shooter, I haven't found that the 300win mag can do anything that a 30-06 cannot do with SAAMI level rounds in 150-180grn bullets.

Frankly, there's no real world advantage for the 300win mag over the .30-06 anyway, unless you're hunting "too big for 30cal" game and shooting rather heavy bullets where extra case capacity really starts to reveal itself. So emulating the 300wm with a 3006 just doesn't make much sense anyway.