Why is my primer so flat with BP?
The loads both 45 Colt, were shot from a Ruger New Vaquero 5.5” bbl.
Load 1) - Starline brass, 260 gr 20:1 alloy bullet from a LEE 255-45-RF-DC mold, sized .452” with Emmerts Improved lube, 35.0 gr GOEX 2fg BP, CCI 350 Magnum primers.
Load 2) - Same components except for the powder which is 9.0 grs of Hodgdon’s CFE-Pistol.
The cases for both loads have had the primer pockets and flash hole uniformed.
The condition of the spent primers differ between the two loads.
The smokeless load, primer edges are still rounded, average MV 925 fps.
The BP load the primers are flat with slight rivet heading at the edge, average MV 823 fps. Outdoor temperature about 22* C (room temp).
Why the flat primer with BP?
Both loads should be under the 14,000 psi SAAMI pressure standard.
I was thinking that the load with BP the primer flash jet has to bore its way into the cartridge case through compressed powder. So maybe a lot of primer pressure is reflected back into the primer slamming the primer back against the firing pin bushing.
The smokeless load the jet of primer gas has some where to go straight into a mostly empty cartridge case. This acts as a pressure relief so the primer doesn’t get thrust back as hard there by leaving the primer with rounded edges.
Any thoughts on why the BP primers are so flat?