I initially thought this to be too overbore, but it is quite close to the 257 Weatherby and 264 win Mag in case capacity to bore ratio, so it is feasible. A 100 gr bullet should reach 3400 fps if you use a 28 inch bbl. Burn rate from R25 to AA8700 is appropriate. Resist the temptation for a shorter bbl as this will quickly drop you into the 240 Weatherby velocities.
Consider using a custom bbl with a fast twist rate in the 1 in 8" range so you can test out the 107 gr match bullet used in 1000 yard competition. What you really have is a more potent version of the 284/6mm that is very successful in that application. The coated X-bullets from Barnes at 85 grs should do well also. For varmints stick with the ballistic tip offerings from Hornady and Nosler but don't go below 70 grains as overstabilization will be a problem with that twist.
I have seen post on other boards with pictures of Winchester's next offerings: a .243 WSSM and a .223 WSSM. I think if you want to build the 6 WSM go for it but the 2 rounds that you compare it to are overbore. I really don't believe you would get much more velocity with the 6 WSM than you would with the WSSM but you would use more powder. Just my thoughts good luck with your project. Sean
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