I understand your point and in some ways you are correct. Especially from a SAAMI standpoint. Now lets talk real world reloading as done by most guys who don't have QL on their IPHONE 16 and a strain gauge in their rifle case.
You can run a 47 to higher pressure than a 260 without showing pressure signs. Doesn't mean jack crap about PSI, just what it shows.
QL will tell us that we are going nuts but the brass won't. So yes the 260 will run 100+ faster than the 47 with similar pressure. However there are plenty of guys running a LOT more pressure than 260 guys are due to brass construction. A 260 will show pressure before a 47 in most cases has been my point all along and I don't think anyone who has worked with both extensively will deny that.
We aren't talking about measured pressure here but pressure SIGNS. Lots of guys outrun their cartridge design and never know they are doing it.
Brass toughness and primer construction are a HUGE factor in pressure signs. I have built a lot of rigs that will show no pressure at what I know is well over the rated PSI and others that cry like a lil girl well under SAAMI, all depending on the brand of brass. Throw in a small flash hole and super tough brass and you have a perfect storm for guys to overload the 47 to 260 specs and have nno clue whats going on. May not be smart but it happens every day.
What I am saying isn't somewhat true, or partially true, it's absolutely true.
There is only so much powder you can stuff in a given case and the 6.5x47 case volume is about 2gr's less than the 260.
If you look at load data many of those publishing data today give you a % number. That percentage is percentage of full case volume.
It doesn't matter what caliber you're talking about they all have the same limitation.
There are lots of ways to deal with pressure from the powder you choose, primer, throat length, bullet type, and seating depth. None of them will make a case hold any more powder other than seating depth/overall length and any tricks you can play with one, you can play with the other.
You're not going to make the 6.5x47 run with the .260 unless you are way overloading the first while under loading the second or by running a lighter bullet with the Lapua than the Remington.
Same bullet same loading philosophy, same outcome, the .260 and the CM outrun the Lapua.