I recently tested Win. and CCI "Maxi Mag" ammo through a chrony out of my Marlin 25.
Both brands gave a 50 point deviation in FPS!
The average velocity of the Winchester 40 gn was 1806, average for the CCI was 1880.
The Win shot slightly under 2" and the CCI slightly over 2" at 100 yards.
With the Win I shot 3 shots into 1/2" and then the next three opened the group up to about 2", this was probably a freak accident but I would like to think it was barrel heat.

Jeremy

I don't think it was bbl. heet at 100 yards wind will cause groups to open up. I have a Ruger the winchester work the best in it at 100 yards on a still day 3/4 in. groups. But if the wind is blowing they will open up to 2 inches like 15 mph. wind.