Pretty certain your American Eagle 158gr SP at 1240 fps was tested in a revolver, probably using a 4"-5" barrel. Most times when the manufacturer has identified the firearm used, it is close to that. Also noticed that almost never is a .357 round marketed as "rifle" ammo; always as handgun ammo. So that makes sense, too.
Your comments on "fast vs slow loads" adds to the complexity, which Gunney noted. There definitely are many factors involved. I'm limited in that my "reloading" is solely done by using manufactured off-the-shelf ammo, which comes with limited info (specs) on the cartridge's build and capabilities. That has always made ammo selection challenging, and yet interesting at the same time.
Appreciate your suggestion on the chrony. I'll check with a couple of friends, who might have one.
PT7
clovishound wrote:...most accurate way to determine...couple shots through the crony will give you the skinny...
FWIW, I ran a 158 grain soft point American Eagle through my 20" BBS. Box said 1240 fps out of (presumably) a revolver barrel. Chrony said it was 1800 fps.