Re: . In past tests the STW proved to produce about 47 fps more veloci
Thats exactly what I did with the 7mm RUM case, same exact length as the 300 RUM. I only tested the 156 gr ULD RBBT and the longer version topped out at nearly the same load as the shorter version, only thing is that load density was lower because of the higher case capacity of the longer case.
Velocity output for the 156 gr ULD RBBT anyway was identical to the shorter cased design only again with lower load density. This did suprise me to see this.
The 257 Allen Mag loaded with a 156 gr ULD RBBT to top velocities with WC872 is running right at 85% load density. With the lighter 130 gr pills I can get her up to around 95% load density.
With the 156 gr ULD RBBT, I have a theory that the 257 Allen Xpress will darn near equal the performance of the larger 257 Allen Magnum because it holds about 15 grains less powder but with the usible powders we have available to us, I suspect I will be able to get right up to 100% load density with the 156 gr ULD RBBT which should still result in nearly 3300 fps in a 30" barrel.
The 257 AM will get 3400 fps if I run it really hard but with this case volume and this very large 25 cal bullet, Our slowest ball powders are just a hair to fast in burn rate for anything much over 85% load density. Still they work very well at that point but I believe this is the largest practical case capacity you can use with this heavy bullet weight and the powder selection we currently have available to us from my tests anyway.
Remember that the standard 257 AM case capacity is several grains larger then the 25-300 RUM because of the improved case design.
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