First shot is slow.

[Wyatt Smith]

The other day while working up a load for my 357 magnum while using 160 grain gas checked semi wad cutters with accurate No.9 the first shot of a string would always be about 100 fps slower than the rest.
After reloading the revolver, it would happen again.
I tried two different bullet styles and several different charge weights and had the same results. It’s a pity too because the subsequent shots were very close in velocity and quite accurate.
I don’t think it’s a chronograph issue as the recoil and report were noticeably less. Also I tried shooting some from the same chamber, so this eliminates a slow hole.
My W296 and IMR 4227 loads perform as they should.
Any thoughts?

 

[Grumpy gumpy]

Do you have a digital thermometer, take a temperature reading of the barrel/chamber before, during and after shooting, could be as simple as temperature sensitive powder and you could be right on the edge of its "sweet spot" Aside from that I have no idea.
Gumpy

 

[Wyatt Smith]

Do you have a digital thermometer, take a temperature reading of the barrel/chamber before, during and after shooting, could be as simple as temperature sensitive powder and you could be right on the edge of its "sweet spot" Aside from that I have no idea.
Gumpy

I may try it, but I can’t see it cooling off enough in the time it takes me to reload. Also they don’t keep getting faster, it levels off. For example one batch I tested the first round 1185 and the remaining 5 were all really close to 1300. If I excluded the first shot the extreme spread was only about 40 FPS. As soon as I got it reloaded, the same thing happened on the next cylinder full.

 

[Nhoro]

Are you firing the first round single action and then DA on the rest ? Maybe it is a light primer strike on the first or something similar.

 

[Ontario Hunter]

Try some 38 Special. See if you get the same anomaly.

 

[Wyatt Smith]

Are you firing the first round single action and then DA on the rest ? Maybe it is a light primer strike on the first or something similar.

Try some 38 Special. See if you get the same anomaly.

All shots are fires single action, but a light strike makes as much sense as anything.
My other 357 loads with two different powders are just fine.