I have 2 ea S&W 640's and Chrony results follow for both same ammo box, same day, same distance from muzzle (10 feet) and perhaps 10 minutes apart.
640 #1, WW 158gr SWC HP+P, ammo over 5 years old, 5 shot average = 814.2 fps; new box, same ammo av = 821.8
640 #2, same as above. old ammo average = 793.8 fps. New box ammo average = 792.8 fps
Both S&W revolvers are 1993 and 1994 made. Old ammo was stored over summer in 112 - 114 degree heat (w. Phoenix in storage shed)
Bought new ammo to see if heat damaged old ammo performance.