Home Forums Hunting & Shooting Ask The Gunwriters Bimodal Velocity Distribution

With such loads a "bimodal" result also can occur due to bullet "pull," the tension from the case neck--unless we select cases carefully for neck thickness, then anneal, size and perhaps even crimp them as exactly the same as possible.

In all the loading manuals going way back from Hodgdon, Winchester (when 296 was different than H110)and Accurate Arms, they always said not to reduce loads and use "heavy for caliber" bullets for best results. Magnums primers also. It is the characteristics of the older slow ball magnum powders for straight wall cases. Blue dot also, but that powder has some erratic results. Enough to be an issue in my book.

Powder Position
Any cartridge containing less than a settled full case of powder may produce drastic differences in pressure depending on powder position. The industry standard requires that powder be gently positioned near the primer end by using the following procedure:
1. Ammo is placed in a loading block with primer down.
2. Grasp test cartridge between thumb and forefinger, lift from block, pause, rotate cartridge slowly in a vertical plane until the primer is up, pause, and rotate until the primer is again down.
3. Seat the cartridge gently into a horizontal chamber keeping the primer end as low as possible.
This procedure is commonly called the "SAAMI Twist" and is followed religiously. Typically, powder positioned at the primer end will give higher pressures and velocities, but exceptions have been noted.

I don't have a copy of the article mentioned so can't help there
I did run across the slight of hand discussion which I believe to be the core of the magazine article but I may be completely wrong. Many people have established position sensitivity with their own gear in their own guns without much publication beyond saying it's there, it matters, powders with a good reputation for the application used at their optimum pressure range and reasonable fill range are likely to work just fine CEM.
Ken Oehler
Quote:
Many years ago while working on a special project, I needed handgun bullets traveling near the speed of sound, approximately 1140 fps. I chose to use a Contender 357 already fitted with a strain gage, light bullets (125 grain from memory) because I don't like recoil while testing, and a batch of 38 Special cases because they were handy. The books said that Unique was probably appropriate. I juggled cavities on my powder measure until the velocity was "about right" before loading a few hundred rounds for testing. (As I recall, the charge was approximately 6.5 grains filling roughly 70% of the case volume.)

During the tests of the equipment I noticed that if the powder was positioned forward at firing, then the velocity was subsonic. If the powder was positioned to the rear, the velocity was supersonic, leveled out it was approximately sonic. That was quite handy for my tests, but it's not what you want for a uniform load.

After my other testing was over, I decided to look at the powder position question and hooked up the pressure instruments. For years I'd read in the SAAMI procedures that you must take care with powder position during tests, but had thought that it wouldn't make a really significant difference in average pressure or velocities. The measured differences shocked me. With the powder forward, I would consistently get near 1100 fps and 18K psi. With the powder back, I'd get 1400 fps and 28K psi. Considering that the SAAMI max for 38 Special is 18K psi and the nominal average for the proof loads is 27K psi, the differences observed in the same load, same case, same bullet, same gun, same day, same instruments, same whatever are very significant.

I didn't do the experiment just one time, but used the same load as an example during several workshops. I'd let two participants each choose five round from the same box of ammo. I'd fire the five high pressure rounds first and then the five low pressure rounds. With the benefit of a little clumsy gun handling and some distracting BS, it wasn't obvious to the observers that the first five were with powder to the rear and the second five were with powder forward. The velocities and pressures always came out as I'd expected from previous tests. It was easier to convince the class that the differences were caused by the cold sweaty palms of the second sucker.

Moral of story, unless you actually measure pressure in your gun, with your components, under your conditions, then you don't know pressure. Even it you think you measure it, you're still not sure.
Ken Oehler

There are stories of different results from the same load assembled at home and at the range because the powder settled on the trip. Another aspect is what Brownell calls porosity of the load. At one extreme, compressed into a single solid pellet the charge should act as an extreme slow burner, (see Richard Lee on seating bullets deep) at the other extreme maybe flashover or even the secondary explosion effect that may or may not exist with drastically reduced loads.

With the velocities barely subsonic, I'd suspect chronograph interference caused by the muzzle blast wave just ahead of the bullet. Sometimes it trips the screen and sometimes it doesn't. This error is usually detected when you use a chronograph with a third screen.

The only known cure is to use a blast baffle between gun and chronograph screens.

With the velocities barely subsonic, I'd suspect chronograph interference caused by the muzzle blast wave just ahead of the bullet. Sometimes it trips the screen and sometimes it doesn't. This error is usually detected when you use a chronograph with a third screen.

The only known cure is to use a blast baffle between gun and chronograph screens.

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