In general roughly 2400 to 2600fps in a 24 inch barrel and roughly a 28 fps loss for each inch of barrel loss from that 24" benchmark.
Example Hornady indicates 2580 fps from a 24inch barrel. same load from a 16 inch barrel is yeilding 2350 FPS. 2580 - 2350 = 230 fps diff / 230/8in diff in barrel length = 28.75 FPS per inch of barrel.
Different barrels, bullets and powders may vary some yet this should get you in the ball park