Estimated case water capacities:
.22 short = 3.1 grains H2O.
.22 Long = 4.3 Grains H2O.
.22 L.R. = 4.4 grains H2O.
.221 Atkins = 5.5 grains H2O.
.25 ACP = 6.7 grains H2O.
A .25ACP case, of .276" O.D. and .604" length will hld from 5.00 to 5.31 grains Weight of Black Powder depending on the granulation size.
Normal load length with a 50 grain FMJ RN bullet is .910" maximum.
if a load of ffg Black Powder weighing 5.00 grains is compressed under such a 50 grain bullet to an Overall length of .850", the Black Powder will be compressed to nearly a solid column, at .840" overall length, it distorts the case, locking it inn the shell holder to the extent i had to use a hammer and punch to remove the cartridge form the shell holder.
I have NOT fired any of these experimental Black Powder loads across a chronograph.
I have fired Factory PPU and Fiocci .25 ACP through my Custom Ruger Single Eight across a chronograph.
PPU states their ammo is loaded to 775fps per CIP Testing and Fiocci says theirs is loaded to 800fps from a 4 inch barrel in their CIP testing.
From my Ruger Single Eight revolver i get average 771fps and average 745fps, probably due to the cylinder to forcing cone gap venting. I figure about 2.1 percent velocity loss due to venting and not recovered by my longer, 10-5/8" barrel.
Based upon B.B.T.I. tests on three types of Factory .25ACP ammo, I should see about 1000fps out of a 16 inch barrel if fired from a NONE Venting firearm. Above 16 inch length they saw a decrease in velocity of the Factory ammo, dropping to around 900fps at 18" length.
Chev. William