My chrono observations, .357 Mag. & .405 Win.


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Crashbox
June 15, 2013, 09:39 PM
Today I went out and tested my regular .357 Magnum paper-punching rounds using Herco, Missouri Bullet 140 TCFP and Federal 200 magnum primers. What I found was interesting.

The extreme spread was quite large, even with individually-weighed 7.5gn charges (131 FPS). And out of 30 rounds each, the standard deviation was 30 FPS weighed, 41 FPS with thrown charges. HOWEVER, when the first round of each cylinder-full is omitted (GP100 4" six-shooter), the standard deviation drops by about 40%; 24 FPS and 23 FPS, respectively. This is telling me something is going on with the first round, and I suspect it is because this powder is position-sensitive. I tend to tilt the revolver toward the ground when loading which will move the powder toward the bullet. After the first round, however, the powder is most likely pushed back toward the case head due to recoil. The two times that I made an effort to load with the firearm tilted somewhat upward revealed a first shot that was notably more consistent velocity-wise with the others.

Running them at 8.0gn of Herco revealed a similar trend, though my sample size was only 20 rounds.


I also shot 15 rounds of .405 Winchester ammo in my Ruger 1-H Tropical using 56.0gn of Reloder-15, Hornady 300gn FP Interlock #41050 bullet and Federal 210 primers. An extreme spread of 114 FPS (1967 to 2081 which is rather anemic IMO) and a SD of 35 FPS. Methinks this is not the best combo for this particular rifle.

A few days ago I clocked seven rounds of .405 using 57.0gn of IMR 4320 with other components identical and the ES was only 34 FPS (2161 to 2195) and SD was 12 FPS. It must be noted that I definitely need a larger sample size of this combo before I can draw any conclusions, but the preliminary data is not too shabby. Weather was virtually identical for both days.


In sum, I certainly have some more work to do, especially with .357/Herco. I think switching to regular primers will likely bring the ES and SD down, but we'll see. Like any tool, a chronograph can be very useful if the resultant data is applied properly. And I just LOVE my Oehler 35P.

This is fun.

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rcmodel
June 15, 2013, 09:56 PM
After the first round, however, the powder is most likely pushed back toward the case head due to recoilThe powder stays where is was due to inertia.
And the case, cylinder, and gun recoil backwards.
The powder is pounded foreword against the base of the bullet.

Then the unfired rounds rattle around in the chambers or magazine until everything gets back to status quo again.

So then it stops, and the powder probably splashes back and forth in the case until you point the gun up cocking it, or down slapping it back down on the sandbags.

There is no way to predict where the powder is in the case, unless you very carefully point the gun up, or down, just before firing each shot.

rc

Crashbox
June 15, 2013, 09:59 PM
Thanks, RC. I overlooked that important part of physics.

But that first-shot anomaly still has me scratching my head...

Crashbox
June 16, 2013, 09:13 AM
I don't know how I could be such a total ditz WRT the first-shot issue and the fact that it seems to run 65-100+ FPS slower than the remaining rounds in the cylinder. The answer was smack dab in front of me and I'm certainly old enough to know it.

Herco- being an older formulation- still uses vacuum tubes.

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