My best guess and that is about it, a guess would be the 6.5×47mm Lapua was first a European developed cartridge unlike the 260 Remington or the 6.5 Creedmoor which spawned in the US.
The 6.5x47mm Lapua (or 6.5x47mm) was developed with a single reason in mind. They were looking to develop an extremely accurate 300 meter competition cartridge. Yes, it is shorter than its brethren but with a case length of 47mm the cartridge is a roughly 63,000 psi cartridge. That is quite a bit of chamber pressure pushing the lighter bullets in the 139 / 140 grain range. My guess, and only a guess would be that they wanted a chamber well suited for an accurate cartridge capable of high chamber pressure using a well suited powder (burn rate for pressure curve). The case volume gave them what they wanted. Having a greater case volume would have likely defeated their intent. That would be purely my guess.
OK, the 6.5x47mm Lapua was developed as a match rifle cartridge designed to offer peak performance at 300 yards. It was never marketed as or touted as a hunting cartridge.
The 260 Remington was actually designed more as a hunting cartridge and in the early years touted and marketed as such. In my little opinion Remington somewhat missed the boat on that note. Maybe they were busy focused on their 6mm BR, I don't know. It was only later that shooters began to see the merits of the cartridge for match shooting using better than hunting bullets. The 260 Remington being in the roughly 60,000 PSI family.
Something else that you have hit upon several times figures into the game here. The shoulder angles. A steeper shoulder angle for any given case will obviously increase case capacity allowing for more powder and increased velocity but do we reach a point of diminishing returns? In some cases you realize a few hundred feet per second more velocity but does it matter? How far or hard do we want to push the pressure curve?
The 6.5x47mm lapua has a 30 degree shoulder, the 260 Remington 40 degrees and the 6.5 Creedmoore comes in at 30 degrees.
The 6.5 Creedmoor could be described as a .260 Rem. modified with a 0.11" shorter case, a sharper 30-degree shoulder, and less body taper. Full case capacity of the Creedmoor is 52.1 grs. of water, compared to 54.4 grs. for the .260 Rem. (both measurements taken using new, unfired cases).
The above quote was taken from here.
Note the 6.5 Creedmoor was developed as a target cartridge and not a hunting cartridge. So two of the three cartridges were developed primarily as target cartridges with only the Remington 260 developed to aim at the hunting audience. That is not to say any of the three would not serve well in either capacity because they have.
between the creedmoor and 260 remington, it seems the shoulder is rolled up further with a sharper angle on the creedmor, which gives it enough extra case capacity for an average of about 100fps.. but feel free to give your opinion on the 3, if i decided to get behind one of them, id have no problem making cartridges, but first impression is that the creedmoor offers the most velocity, and is better established in the consumer market being able to find more ammo for it than the other two
I would agree with that to a point. I see brass and components just about equally available. Keep in mind that the 6.5 Creedmoor was based on an improved version of the 260 Remington. Bitch of a choice huh? Today? I would try the Creedmoor simply considering the cartridge wasn't around when I had my 6.5mm phase in life.
Ron