If it is true all I can say is...

[size=6]WHAT A WASTE OF TIME AND MONEY![/size=6]

Probably the reason they are doing this.

I might be old and cynical, but I think if the ATF is behind it, it's to cause confusion.  Think of a poor guy with an old Mauser converted to 7.62 NATO or an Indian Enfield 2A that doesn't do research on the rifle.  They might ask for advice for the first batch of ammo they buy, and a kind person points them to 7.62 NATO that is marked as 308 WIN.  That ammo shoots fine for them, then when they go to Wal Mart and buy 308 WIN, because that's what was on the label on their first box, they take a chance on blowing their rifle-up and possibly injuring themselves or an innocent bystander.  Creating confusion over 7.62 NATO and 308 is a very bad thing.  If the ATF is encouraging remarking the ammo, then I think that is criminally negligent.  Harassing gun owners with ridiculous interpretations of the law is one thing, but risking our health is another.

Since I someday aspire to own this caliber, is this a case-size difference, powder, or what?

No, it's not a case size diffrence, the commerical ".308 win" is loaded hotter than the nato 7.62, example, you could shoot nato 7.62 in a 7.62 indian ishpore enfield all day long, put some commerical .308 in, and look out!! it may blow up on you, or stretch the action to the point of being unsafe to fire, one just has to careful of what excatly you're putting in you're rifle! Same goes for those Spainish Mausers in .308, (FR-7's) these FR-7's are small ring mausers that were rebarreled for 7.62 nato,(they were at one time 7x57) and are being sold in this country as .308's Frankly, I think these are dangerous rifles to shoot with either the 7.62 nato or the commerical .308, as both will push this action beyond it's pressure limit, Maybe, look at a FR-8, these are spainish bolt actions built on a mauser 98 (large ring) design, and should be able to handle 7.62's or .commerical .308

Can anyone tell me where the .308 is more powerful than 7.62x51 comparison using identical pressure testing methods might be found?

I have seen numerous websites showing this info, and they always refer to the 50,000 psi figure for 7.62x51, and the 52,000 psi figure for .308 commercial ammo, but never have the actual "original" test equipment or documentation (usually they reference some other source for this).

Never is there any discussion on the method used to measure this pressure level. You cannot compare apples and oranges as the old saying goes.

I have used both types of ammo and my experience shows that different brands or manufacturers are loaded to different power levels, but there is no definitive "one is always stronger than the other".

Some examples: In my .308 garand, commercial winchester and remington .308 150 grain FMJ ammo will repeatedly fail to fully cycle the bolt. The bolt short strokes just about every single round. This does not happen with the 7.62 147 grain FMJ that I use (Hirtenberger, portugese, and US (winchester)surplus). Also the 3 grain difference in the bullet is meaningless. If you pull the bullets and weigh them there will be a 10% or more fluctuation in their weight (150s will weigh between 146 and 154 grains, same thing for surplus).

Tried all types of ammo in the FAL. The gas setting will also need to be lowered (closed) to cycle the above mentioned commercial stuff.

My Ishapore Enfield has never had a failure with using the "high power commercial" .308, and yes I regularly check the headspace with gauges.

My first .308 surplus rifle was a spanish FR-8, and it has over three thousand rounds through it of a mix of the various commercial and surplus stuff. No failures there either.

btt