Loading phenomenon.

[Bwana]

The phenomenon I refer to occurs on rare occasions and yet I have experienced it often enough over the years to not be surprised when it happens.
What I refer to is the rise, then plateau, or drop, only to rise again, in velocity when working up a load with increases in powder and everything else staying the same.
This seems to occur when using mainly slow powders in large capacity, for caliber, cases; ie, 357mag, 44mag.
My theory concerning this is that the powder is loose enough initially to fire a fair amount of the powder increasing the velocity as the powder charge is increased. At a point the amount of powder is such that the amount of powder "fired" by the primer is retarded by the density of the powder column. This results in the plateau and or drop in velocity. As the powder charge is further increased the powder density is such that the amount of pressure developed allows the proper burning of the rest of the powder and the continued increase in velocity on the chronograph.
None of these load series have resulted in undue pressure levels or "hangfires". I have not tried to replicate the results when encountering them as I knew what was happening and it was like "gas", it would pass and amount to nothing. Ambient air temperature seemed to play no part as it occured over a wide range.
So, I thought I would throw this out there and see if anyone else had encountered this phenomenon and invite your comments.

[stubshaft]

I haven't had this occur in my loads. I usually see a somewhat linear velocity increase the a plateau where the addition of more powder brings increasingly smaller gains in velocity.

[felix]

Yes, it happens, and is an indicator a change in powder is warranted. Even the same powder, different lot might be just what the doctor ordered. Also, different gun, different results. Things become much more predictable/consistent when the gun becomes more and more of the BR type. ... felix

[44MAG#1]

I have had this happen too in a 45/70 with H4895.
I would be interested in the reasons too.
Felix may have the answer.

[subsonic]

I have seen it before too, but dismissed it as possibly mixing up load samples or poor powder weighing, chrono errors, or just being within the normal variation, etc. I have never tried to duplicate it. It is not a huge velocity drop when it does drop. I cannot remember exactly what loads did it, but it does ring a bell now that you mention it.

[Larry Gibson]

I've seen this phenomenon in very rare circumstance. It can only be counted if it occurs with consecutive tests conducted correctly with a sufficient sample size of each test string. Other wise the conditions can be too varied; I.e. testing different loads on different days withweather conditions and chronograph set up being different. Even loading the "same" load or the "same" test loads at different times can easily skew the results.

An insufficient sample size is also needed or the statistical "assurence" is not high enough for a valid conclusion. Three shot test strings tell us little to nothing. Five shot test strings only give an idea and no statistical level of assurence. Seven shots test strings are the bare minimum with the SAAMI standard of a ten shot test strings being the best. We then also must test 7 - 10 test strings of the same charge (probably one of the middle range charges would be best) consecutively to get a small idea of the ES between different test strings of the same charge, all other things being equal. Anyone who has ran 3 consecutive 5 - 10 shot chrono strings of the same load knows the average velocity, SD and ES will very seldom be the same. They may be close but there almost always is a difference.

If we conduct say 10 different 10 shot test strings loaded consecutively of increasing powder charge on the same day under the same conditions and tested on the same day consecutively under the same conditions and we observe the phenomenon they there may be some validity. Otherwise it is apples and oranges.

Larry Gibson

[williamwaco]

+ 1 on the sample size comment from Larry Gibson.

Now my personal experience. I occasionally see a load register less ( or more ) velocity than I was expecting based on prior loads with the same powder and bullet. ( Meaning the same can of powder, same primers, same cases, same box of bullets. )

What's up with this?

Every time this has happened to me, a carefull inspection shows that one of the sky screens has jumped out of its indent and the distance between the sky screens has shifted.


Cheque it out.

.

[felix]

The statements above are all correct. Powder rotates as it is burning, following the circle down the drain phenomenon. How that starts is based upon the powder's ignition. The boolit's forward movement follows the powder's generating pressure acceleration curve. When the boolit HAD BEEN ACCELERATED faster AT SOME POINT than that of the pressure's volume expansion, the boolit begins to loose its acceleration. If this occurs under all conditions, the ignition of the gas volume must be more gentle, or enough energy left in the powder to maintain a smoother volume acceleration curve. The powder that Larry suggests using now in the 30-30s might be just the ticket, provided if the speed range is correct for the gun-load on hand. It's called Hornady's LeverRevolution powder? ... felix

[Rocky Raab]

Felix, I'm not convinced about the "rotating powder" theory. In fact, that's the first I've heard of it, including from factory ballisticians. I'm not discounting it, but I also fail to see how it correlates with the phenomenon being discussed.

We must not overlook a far simpler and more common variable: bore fouling. Fouling can raise pressures even though velocities are reduced. Under the right conditions and timing, such a result could appear in the middle of an increasing charge test string - thus giving the illusion that something odd is happening.

[303Guy]

I have found something along those lines of the denser powder column slowing ignition. That was with the hornet and Lil'Gun. I was compressing the powder to get enough in the case with the end result of a somewhat high loading than would be expected with the powder charge. That powder column was compacted and hard to dig out so it was likely constricting itself which would have helped with consistant burn in the absense of any neck tension at all. So to me it seems likely to be as Bwana and/or felix have hypothesized. Perhaps a bit of both. Rocky Raab could be right too as another possible cause or perhaps contributing factor. This is an occasionally observed phenomena, not a tested and proven fact - yet. It could be tested if such an observation is made again by duplicating the conditions and cleaning the bore in between shots to test for fouling being responsible. It's a good excuse to do lots of loading and chronographing.;-)

[stubshaft]

Just goes to show that even an old goat like me can learn something new.