BFO,

The figures don't add up with the .577. With 160 grains and a 645 grain bullet at 1650 FPS, and 4000FPE. I just don't think 160 grains is enough go powder. At 130 grains ffg the 12 bore is barely making 1250 fps with a lighter bullet. I'm not sure 160 will take it to over 1600FPS, it might seems every 10 grs gets another 100fps ROUGHLY.

Any body have a .577 Express in BP that can confirm?

Colorado