Like Eddie said, it is offered in SA and LA. It is based off the .257 Roberts which was derived from the 7X57 Mauser case. It (7X57) originally started life in a long action. I wouldn't think there would be any benefit to go SA if you were thinking of long for caliber, ballistically effeciant bullets for long range.
All three of these cartridges have enjoyed good success. I say "good" because like alot of other "good" cartridges they have been somewhat left behind. The shooting industry folks have introduced and updated as they saw fit. The 7mm-08 does everything the 7X57 does, the .25-06 (25WSSM) out classes the .257 Roberts. The .243 does not out class the 6mm ballistically, but Winchester got a foot hold on the market by introducing the .243 with the right barrel twist.
It (6mm) would probably make a good cartridge necked up to 6.5. It may have already been done. .264" is right in between .257" and .284". The case is slightly longer than the 6.5X55 Sede, which is a great cartridge, so with that you could probably get a little bit better ballistics from it.
You already have the .260Rem, 6.5-06 (and AI vers.), the 6.5X284, 6.5BR, 6.5RemMag, .264WinMag. I think there's also other wildcats out there (6.5 STW) that could provide the same or better level of performance.
Or if you just dead set on a 6mm necked up to 6.5, go for it. Either route you choose, have fun along the way. Happy shooting! JohnnyK.
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