Do you think if you'd given a bit more information in your header, you have got a better response?
I certainly can't answer your question but hope you find what you're looking for.
The .264 is meant for the American market, and the 6.5 is for the European shooters!
Seriously though, I haven't the fantest idea, but please let us know hene you find out.
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saeed@ emirates.net.ae
www.accuratereloading.com
Two different wildcatters may have made "almost" the same round, with slight, but important, differences. So, they are named differently to avoid using the wrong ammo in the wrong chamber kind of thing.
When the 260 Rem was still a wildcat, it was the 6.5-08. Some guys had different case neck lengths. Using a too long neck in a chamber cut shorter could have serious downsides to it! So, you'll still see 6.5-08 & 260 reamers. Yes, very close, but different enough to matter.
The 264 UM & the 6.5 UM may have slightly different necks, slightly different shoulder angles, etc.