Dave,
When Richard and I were playing around designing the 7mm AM we knew from the start that we would be trying some extremely heavy bullets in the 225 to 250 gr wright range. As such, we designed the barrels with Dan Lilja to hopefully handle bullets from 200 up to 250 gr and as such we went with the 1-7 twist to hopefully cover everything.
This is generally not the best way to do things but I could not afford tooling for two different twist rates at that time.
It has been proven that the 200 gr ULD RBBT will stabilize in a 1-9 twist at velocities over 3100 fps. In fact the 1-7 actually will limit the top velocity of this bullet because it tears it apart in rounds such as the 7mm AM. This produces a velocity ceiling of around 3200-3225 fps, typical of any J-4 bullet in a fast twist barrel.
My personal 7mm AM that I am getting ready to build will have a 1-9 twist 6 groove barrel which I plan to use with the 200 gr ULD RBBT.
With the new 250 gr ULD RBBT, the velocity level will be significantly lower then anything that can be produced by the 200 gr ULD RBBT. As such we will not have to worry about the high RPMs like we do with the 200 gr ULD RBBT in the 1-7 twist barrel.
The 1-7 will also be MUCH more appropriate for this length of bullet as well.
In all honesty, I am looking for a 2900-2950 fps velocity range with this bullet and the BC should push 1.000 pretty easily from what I have seen. If we hit that and the bullets stay together, ballistically it will offer the same BC as the best 50 cal match bullets but with a 300 fps velocity advantage.
We are not really looking at bullet drop here to improve on. More interested in wind drift or limiting it as much as possible and retaining as much velocity as possible for 2 mile shooting.
Will it work. How knows!!! We will soon when bullets start to fly!! If it does not at least we proved it did not work instead of just thinking it was not a good idea. If it does work, we will open up a whole new relm of ballistic performance for sub 338 cal cartridges.
Kirby Allen(50)