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I have been running mainly 10 grains of IMR Trail Boss under this bullet. My average velocities are hovering only in the 840's FPS. I have read where using a 270 SAA bullet has reached 1,000 FPS with the same powder from the same gun. What am I doing wrong?
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Odds are you are doing nothing "wrong". Same charge weight under a heavier bullet will produce higher pressure, and higher velocities. If the 270gr seats deeper than the 260 gr pressure will increase also due to decreased case capacity, thus equaling higher velocity. Could be bullet profile or could be your not getting enough pressure to expand the bullet base and seal the bore. How did you come up with the start load? Trailboss can completely fill the case below the bullet and still be safe. As others have said, TB is not know for velocity and in big bores is generally used for bunny fart loads. Even Unique should give you well over 1000fps with that bullet.