Hello BongoBoy, glad to see you here as well.
I had spoke with Andrew on another forum about initally starting at 8.4 grains of 800X to mimic the load Underwood was using under the 220 cast which is the same length as the 155TAC XP and would occupi the same case capacity. That would be where I would be for performance and pressure...
Like I mentioned the same weight lead core bullets are way shorter than these 100% copper by weight.
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