I thought the link I posted was useful, but I found different information in "Reloading for the M1 Rifle", by John R. Clarke. He says -
...When reloading .30-06 ammunition for use in an M1 rifle, there are two things that must be considered over and above normal considerations. First is that the M1 depends - as the manual says - on the gas pressure from one round to operate the mechanism for the next shot. This pressure is delivered through a port near the rifle's muzzle to a piston to operate the mechanism. The pressure needed to do this is 6000 p.s.i. +/- 2000 and it is usually referred to as "port pressure". Though there are other factors, port pressure is largely influenced by the bullet being loaded and by the burning rate (or relative quickness) of the propellant powder used. To attain usable port pressures in a .30-06 M1 requires use of a powder with a relative quickness between IMR 3031 (the quickest) and IMR 4320 (the slowest). Powders that are "faster" than IMR 3031 will not develop useful velocities within safe pressure levels. Powders "slower" than IMR 4320 can develop useful velocities but generally develop excessive port pressures. Excessive port pressures in an M1 need not be a dangerous condition (though it can be). Excessive port pressure will damage the rifle sooner or later, by causing the operating rod to bend. It may also cause failures to feed, wherein the bolt cycles before the magazine follower can lift a fresh cartridge into the bolt's path....
Referring to the powder burn rate chart, AA 2520 is right above IMR 4320 and is therefore "OK" as far as Mr. Clarke is concerned. The other article described IMR 4064 as the slowest powder.
These powders aren't separated by much in the list, so maybe these authors are splitting hairs.
When I researched this a few years ago, I couldn't get a definite answer on whether Varget would work or not, so I chose to use an adjustable plug as a solution.
It looks like Varget would have worked fine without the adjustable plug.
Summary? Information on this topic isn't consistent. There is a potential problem if the wrong powder is used, but there appears to be quite a selection of "right" powders...