What kind of velocities could one expect in a 300RUm necked down to 6.5mm with 140 gr bullets. Anywhere near 4000fps? Ballistics are phenomenal with the .618 ball. coefficent a-max at 4000 fps. Also, any idea of barrel life?
Just kickin it around for a super dooper 750 yard groundhog gun.

travis20032003

I have built and pressure tested three 6.5 STW rifles. The RUM case is about another 15gr of H20 in capacity. With the 140gr bullet I got 3350fps with a 28" barrel and around 3500fps with a 32" barrel(around 65000psi). My guess would be that the 6.5RUM might go 3600fps with a long barrel(32"). Simulations with QuickLoad indicate about a 100fps difference. At this amount of case capacity there are diminishing returns. As one person put it in "Precision Shooting at 1000yds.", "this was an exercise in inefficiency".
The reasonable powder selection would be limited to the slow surplus ball powders. Even Retumbo would leave a lot of empty space in this case and therefore not take advantage of the added case capacity. There would also be excessive fouling as the STW starts increasing pressure after 10rounds.
You would not even get close to 4000fps with a 140gr. bullet even if you shot at 75000psi which is not recommended. That being said, you would still have excellent external ballistics.
As you can tell I have given this possibility some thought but still can't bring myself to do this project.

the 7 RUM with the 140 will do about 3550.the 6.5 would be a little slower with the same bullet wieght.

A friend of mine built a 25 RUM, for deer hunting this year. Pretty amazing. Personally I'm happy with alot less.