Primer setback on an empty cartridge

[riflemon]

Folks, I have a question I'm hoping someone on here has a better answer than what I came up with. I had a couple of 45 LC empty but primed brass that I decided to cycle through a revolver to demonstrate something without putting dangerous lead out the other end. Both were from original factory loads with the bullets pulled. One was S&B (Sellier and Bellot) and the other was Winchester USA headstamp brass. When fired, the Winchester experienced profound primer setback, enough to make cycling the revolver difficult, and requiring disassembly to remove. The S&B had no such problem. What I found surprising was that the primer was setback at all, when there was basically no back pressure on either cartridge, say from powder or a bullet.

I have fired a number of both of these factory loads and never experienced a primer setback with an intact cartridge (powder and bullet in place) so why does the empty case experience this? Is there something about the resistance provided by the intact case that actually lowers the pressure? Seems counterintuitive to me.

Thanks for your thoughts!

 

[LaneP]

Empty primed cases will almost always experience primer set back, depending on the amount of headspace. In a normal firing sequence, the pressure of the expanding gas forces the case tight to the chamber walls, and the head then stretches back, reseating itself on the primer. That function is missing with the primer by itself.

 

[JJFitch]

Perfectly normal as mentioned above!

 

[FROGO207]

Yeah when ignited the primer will set back then the round builds pressure and pushes the brass back onto the primer. This will not happen if there is no propellant or bullet to build pressure. The S&B brass has a tighter fit and the primer stayed put. The reason some choose not to reload that brand of brass. Have to push hard to seat primers and some have problems doing so.

 

[riflemon]

Alright, I knew you guys would be able to explain this! I had an intuition that the lack of normal pressure had something to do with it but I didn't grasp the whole normal sequence.

 

[lordpaxman]

@riflemon, welcome to THR!

 

[JFrank]

I’m not a pistol reloader so forgive the stupid question;
How is a straight wall cartridge effected by headspace ?
Isn’t that reserved for shouldered cases ?
Curious

 

[FROGO207]

At least with a revolver there has to be a little bit of slop between the case head and the recoil sheild. If there was none the cylinder would lock up and not turn. A 9MM or a 40 acts more like a rifle cartridge but headspaces on the mouth of the brass rather than on a point on the shoulder. On all these there needs to be a range of length that will work without jamming and still allow the firing pin to do its job.

 

[JFrank]

Thx
Now I know…
J

 

[JJFitch]

I’m not a pistol reloader so forgive the stupid question;
How is a straight wall cartridge effected by headspace ?
Isn’t that reserved for shouldered cases ?
Curious

Straight walled, rimmed cases headspace on the rim. The reason it is rarely required to trim straight walled cases!

Smiles,

 

[jmorris]

What I found surprising was that the primer was setback at all, when there was basically no back pressure on either cartridge, say from powder or a bullet.

The pressure that pushes it out is the “blast” it creates. Many years ago, my Brother and I shot plastic and rubber bullets at home quite a bit. No powder, powered just by a primer, we drilled out the flash holes to a larger diameter just for our no powder loads, to keep the primers from backing out and dragging as the cylinder rotated. It worked but never used the cases for powder & lead loads as the modification was permanent.