Primer bullet push & primer setback

[texasmac]

I was reading an old internet link this morning on testing the power of various primers. I use Federal GM150M match large pistol primers in .40-65 & .45-70 BPCR cartridges. So, being somewhat bored, I decided to determine how far a primer along would shove a 540gr Paul Jones Creedmoor bullet (16:1 alloy) down the bore of my Browning. As the photo below displays, the bullet was shoved almost completely into the lands. The land impressions stop just short of passing the driving band. No doubt using a softer alloy or a rifle primer would have resulted in the bullet being pushed much further into the bore.

BTW, on a related subject, in an article I wrote some time ago titled, Firing a Black Powder Cartridge - Progression of Events (http://www.texas-mac.com/Firing_a_Black_Powder_Cartridge_-_Progression_of_Events.html), I made the following comment:

“As the primer ignites, the force from the primer alone is more than sufficient to drive it backward out of the primer pocket until it contacts the breechblock resulting in primer setback unless the case stretches back, contacts the breechblock and reseats the primer flush with the rear of the case head.”

The above is the main reason the softer breechblock face of some older 19th and early 20th century rifles can experience peening around the firing pin hole when firing cartridges with pistol primers which are not as tall as rifle primers & can gain more momentum prior to striking the breechblock face.

Later on in the article I commented on what happens to the primer when firing a full power load. “Although the case locks against the chamber wall the pressure is sufficient to stretch it back enough to reseat the primer flush with the rear of the case. By the way, if you doubt the case is stretching back to reseat the primer, fire an unloaded case with only a primer. If the primer remains slightly backed out of the primer pocket but is flush after being fired in a full powder load, it’s a clear indication the case is stretching back slightly, but it’s minimal and is not a reason for concern assuming the headspace and/or head clearance is not excessive.”

In today’s little experiment the primer was certainly setback the amount of the head clearance.

https://i.imgur.com/MWeEhgv.jpg

Wayne

[MT Chambers]

According to some, that little bit of head start for the bullet causes inaccuracy, and hence some folks using lighter primers and/or a thin wad between primer and powder.

[greenjoytj]

I have never detected by measurement any case stretch in my 45 Colt.
I believe the whole case slides backwards to re-seat the primers.
Firing BP, the primers are very flattened but with smokeless the primer edge stays rounded and that’s with both loads at full power 35.5 grs 3fg Old Eynsford and max book load 9.2 grs of CFE-P.
I thought straight wall cartridge cases didn’t stretch. Maybe it’s the low 14k psi operating pressure of the 45 Colt that allows the case to slide backwards floating on gasified lube slime that blows back along the case sides.

[M-Tecs]

I believe the whole case slides backwards to re-seat the primers.


That one is easy to test and verify. Just fire primed brass in a revolver. They normally lock up due to the primer setback. Same for the quickdraw wax bullet competitors. They have to drill out the flashhole to prevent this.

[BHill]

Great job on the photography and recovering the bullet without deforming it. Looks great.

[greenjoytj]

That one is easy to test and verify. Just fire primed brass in a revolver. They normally lock up due to the primer setback. Same for the quickdraw wax bullet competitors. They have to drill out the flashhole to prevent this.

Without out the mass of a lubed bullet for the pressure to act upon so that the Newtonian third law of motion of equal and opposite movement which can’t be ignored will take place.
Without a bullet in the case that massless primer test would be invalid.

I say in low pressure cases, the case moves backwards to reseat the primer, not stretch backwards as occurs in high power cartridges.

If low pressure straight walled cases grew a bit on each firing I would be happy because my too short Starline cases would get to grow the trim to length.
45-70 shooters wouldn’t need to buy case stretching devices to correct short cases.

[M-Tecs]

Without out the mass of a lubed bullet for the pressure to act upon so that the Newtonian first law of motion of equal and opposite movement which can’t be ignored will take place.
Without a bullet in the case that massless primer test would be invalid.



The gasses for the primer pressurizes the primer pocket forcing the primer out of the primer pocket with standards size flashhole. That is why blank cartridges and wax bullet cartridge's have larger flash holes. Without pressurizing the case it is an irrefutable test that the primers in fact do back out.

[greenjoytj]

I agree 100% primers back out of the primer pocket. Their launched backwards by the force of the priming compound exploding. In a high pressure bottle neck high power rifle cartridges the pressure inside the case seals the case to the chamber wall so tightly the case can’t move backwards. The case head stretches backwards to reseat the primer. Creating a ring of thinning near the web of the case head. The stretched brass will eventually require trimming to length. The straight walled low pressure revolver ctg’s don’t stretch backwards, they slide backwards to re-seat the primer, aided by bullet lube (if present) gas blown backwards.

I don’t know if high pressure revolver ctg’s like 45 Casull stretch, I suspect they do.