. The ullage provides a certain amount of freedom
for the propellant bed to compress and move during the initial stages of the interior
ballistic event. With minimal ullage available, the propellant merely transmits the
primer force to the projectile, acting much like an incompressible fluid. Conversely,
a cartridge with more ullage will create a situation where the initial motion of
projectile and propellant act more in unison.
Ever since the Brennan demise of Precision Shooting, I have been seeking an alternative to step into the void created. Is Accuratshooter.com the one?
As far as primer behavior upon ignition:
Primer brisance varies greatly between manufacturers.
Brisance changes within the manufacturer separates standard from magnum classification.
Complete burn and utilization of a powder charge is the goal as the solid fuel becomes a gas causing work to happen on the projectile.
Efficient conversion of solid to gas has been experimented upon with extended flash holes. Not practical in small arms but US Army tests found extended flash holes to be promising.
The duplex load has been used prior to the consistent availability of high quality magnum primers. Even then, not all magnum primers are equal. My testing with 120gr of powder per round revealed Federal to be the hottest and most desirable.
The interesting thing they “concluded” (although did not test) was that lower ullage loads (like the H322/77gr load we use) help the primer and powder work together instead of the powder just acting as an energy transfer to the bullet.
The bullet yaw observations when jumping to the lands (which we’re all aware of) was interesting too.
My apologies, I am now fully grasping the gist of this thread. Yes, I can absolutely believe how a reduced load density powder charge with the proper primer can produce good results. Load density that is too light can exhibit erratic ignition behavior including hang fires and even detonation. Traditional logic dictates that the high density loads produce better precision as the propellant is consistently arranged inside the case. But, what happens when the reduced loads are used? The old school nostalgia of service rifle members tipping the fast burning charge back onto the flash hole before firing? But, what happens if a hot match primer is used? I believe what you already know. A hot match primer can in fact go against traditional logic that low density powder charges are to be avoided.
Neither am I. I posted to get some more thoughts on it.
Yes, I can absolutely believe how a reduced load density powder charge with the proper primer can produce good results. Load density that is too light can exhibit erratic ignition behavior including hang fires and even detonation. Traditional logic dictates that the high density loads produce better precision as the propellant is consistently arranged inside the case. But, what happens when the reduced loads are used? The old school nostalgia of service rifle members tipping the fast burning charge back onto the flash hole before firing? But, what happens if a hot match primer is used? I believe what you already know. A hot match primer can in fact go against traditional logic that low density powder charges are to be avoided.
The 77 load I run has higher “ullage”, but not so much that it’s detonation danger. And if I’m understanding their conclusions correctly, yes. They’re saying a high ullage/lower density charge is better. Although they didn’t shoot such a load to conclude this.
I’d LOVE to know what their testing would show with the mild WOLF SRM primer… i.e. does that mild primer push the bullet prior to output charge ignition too? Especially in the context of a higher ullage cartridge…
It’s also unclear what the story is on the #41 primer. They say the legacy primer was milder despite the spec being the same. I’d love to know more on that. Especially since the very current production primers I have are insane hot.
The primer ignition transfers the energy to the rear of the bullet because the powder charge acts like a non-compressible liquid. The mimic the activity with inert powder-like material which isn’t exactly like real ball powder, but it recreates the motion.
Most intriguing article encountered in a very long time. Those of us that attempt to visualize what is happening in those micro seconds of initial ignition appreciate that the ARL has taken a scienctific approach to our pondering. My take aways from the parts I am able to comprehend are what Markm has posted…
Thank you Sir for bringing this thought provoking article to our attention.
Perhaps if these scientists could also define the percentage of affect on the load that the primer alone is responsible for…
I once had the opportunity to question a few of the leading ballisticians at SHOT regarding the energy required to engrave the rifling and the relative affect twist rate made regarding that. Opinions stated were less than 1/2 of 1 percent of the cartridges potential energy. Since we as reloaders on a hobbyist scale cannot control so many factors, we must simply shoot what works.
The best 556 powder thread got me wondering about another factor. I wonder what impact an extruded powder would have on the whole test. We’ve been shooting a bunch of 3031 lately that is the complete opposite of Mil ball powder. I wonder if that powder charge would also act like a non-compressible fluid too.
Finally read the article and I developed the following conclusions:
The shock wave is likely proceeding the particles of combustion. The shock wave is the force that is compressing the propellant AND dislodging the bullet from the case.
The particles of combustion and the bulk of the flame behave as a focused JET as these elements are funneled through the flash hole.
The burn rate of the propellant become heterogeneous as the shock wave is likely distorting the individual kernels of powder. The physical characteristics of the kernels of powder combined with surface treatment is how burn rate is controlled. Therfore, the propellant once it has been crushed has been physically altered before it is ignited by the particles and flame jet.
Cases with low load density may experience the bulk of the propellant conversion into gas inside the barrel and not the case.