Years ago, Charlie Dell did a batch of experiments on primer "power", without much success.  He was propelling air-gun projectiles and measuring their velocity.
My impression at the time was that he was looking for UNIFORM performance of the primers.

CHRIS

All this measures only the explosion pressure, not the heat transfer, which is what lights up the powder charge.

One of the issues you'll run into when trying to make up sub sonic loads is the danger of not sealing the chamber. If the pressures aren't high enough to expand the case and seal the chamber the powder will blow back around the case and may hit the shooter's face.
Back when I shot bullseye matches in our local league I was experimenting with how slow I could make my .45ACP rounds to allow my 185 SWC bullets have as little recoil as possible, and allow me to get back on target for timed and rapid fire stages. I was getting some help from S&W tech who sent me a set of various springs to allow my 745 IPSC pistol to cycle properly with light loads. 
The testing of lighter and lighter loads went great until I reached a load light enough to not seal the chamber, and got sprayed in the face with burnt gunpowder. I picked up my ejected brass and saw it was black with burnt powder. So I stopped at that point and went back to the next hotter load of about 50 fps faster. It had slight blackening outside the case mouth, but never anything getting past the case. 
So if you mess with squib loads, just be sure to wear good eye protection, and even a grinding shield might not be a bad idea as you're working down to lower levels.

Wayne, et.al

Thanks, that is exactly what I was looking for.

So as I start to experiment with breech seating
your information and that offered by others (Val) (thanks) provide some helpful and information on the subject.

Given that approximately .2" of increased case volume
is  relatively small I would assume there is an insignificant decrease in chamber pressure as a result of the primer induced projectile movement. 

 

Bob


texasmac wrote on Jan 20^th, 2023 at 7:43pm:

Bob, 
I also experimented with the .45 Colt in my S&W Model 25 that I used for practical pistol events. Like you I found those very low velocity loads extremely accurate, but also blackened cases when firing them. They never resulted in powder or gas back in my face with a little blackening, and both recoil and accuracy were great at that point.

John,

Thanks for posting the link to the primer pics...and your observations,

RE: "In German's testing he found that the smallest primer flash provided him with the most consistent powder burn / accuracy."

Back when I was experimenting with a 45-70 
(Swiss 1 1/2F)
My lowest ES and SD was using large pistol primers and a news print disk over the primer.

My next experiment will be shooting a 32-40 using 30 American cases with the small primer pocket & opening. 

Thanks again

Bob
JLouis wrote on Jan 20^th, 2023 at 1:43pm:

JLouis wrote on Jan 20^th, 2023 at 11:12am:


See the attached articles which may be the ones your are referring to.
Wayne