Newbie questions - Superlight loads = high pressures? and Charges of powder...


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dandean316
August 26, 2003, 02:54 PM
I was reading in (I think) the ABC's of reloading book and they said you shouldn't go lower than the minimu loads listed because you can create pressures in your gun that are dangerous. Is this right? Seems if you use less powder, you get less of a charge which creates less pressure which launches the bullet at a slower speed. I understand not going *real* light beacause of case ejection and the bullet getting stuck in the chamber etc.

The other thing I noticed in the manuals I have is sometimes a larger bullet will need LESS powder to shoot the same FPS. I am comparing 200 gr. SWC to 185 gr SWC - same design, just less weight. Wouldn't you need more powder to shoot a heavier bullet the same FPS?

And last: In my Lyman manual (the pistol only one), 2 - 200 gr. lead alloy bullets need different charges on the same powder (Bullseye, 700X) to shoot the same FPS. Is the design of the bullet that big of a deal?

Any comments are appreciated.

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griz
August 27, 2003, 12:48 AM
1. Yes there can be situations where reducing the load can cause dangerous pressures. Usually this would be a high capacity case (relative to bore size) that uses a slow powder. Through some mechanism that is beyond me, the pressure can spike very high and do damage. In MOST other situations, not all, less powder is not dangerous except for stuck bullets. You do however reach a point where the pressure never gets high enough for the powder to burn efficiently and so the load never really “gets started”.

2. Typically you use more powder for lighter bullets. Of course the lighter bullet will go faster. In rounds with a large range of bullet weights, the difference is great enough that the optimum powders for the heaviest and lightest bullets might even be different.

3. Depends. Sometimes in pistol cases the nose shape of the two bullets is different enough that the length of the bullets are quite different. Since the overall length of the cartridge is relatively constant, the amount of bullet inside the case varies and changes the effective case capacity. The result is different charges for the same weight bullet.

Hope this helps. As always these are NOT concrete answers. I would encourage you to keep learning and asking more specific questions as you encounter them.

Steve Smith
August 27, 2003, 09:50 AM
I think that the theory is that you have little powder in a large case, and the flame from your primer ignites more powder, along the length of the cartridge, than it would if the powder level was higher. In a medium or heavy load, the ignition would progress through the powder, but if it all ignites at one time it basically detonates.

Bullet
August 27, 2003, 05:25 PM
When I first started loading I noticed the powder difference in bullet weights too. It seemed the opposite of what sounded right to me. The reason lighter bullets use less powder than heavier bullets was explained to me this way - powder when ignited in a confined space creates pressure. In a cartridge the pressure escapes by pushing the bullet. With a heavier bullet there is more resistance to movement (more weight & more contact with the bore). When there is more resistance (heavier bullet) the pressure builds faster than when you have a lighter bullet (less resistance) so you need less powder to achieve that pressure.

Can anyone verify if this is right?

facedown
August 27, 2003, 06:11 PM
Bullet said: "With a heavier bullet there is more resistance to movement (more weight & more contact with the bore). When there is more resistance (heavier bullet) the pressure builds faster than when you have a lighter bullet (less resistance) so you need less powder to achieve that pressure."

________________________

There are additional considerations but that's the basic idea.

Another way to think about it is that with a fixed amount of powder, the lighter bullet will exit the barrel sooner than the heavier bullet thus there is less time for the pressure to build up.

Art Eatman
August 27, 2003, 09:12 PM
About 30 years ago, there was an article in TheAmerican Rifleman, discussing a blowup of a .243 rifle that was handloaded with a light charge of 3031. About 20 or 25 grains, IIRC, rather than the usual 35 to 40. The hypothesis was that "under certain conditions" a light load of IMR powder could cause this problem. Trouble is, nobody is sure about what conditions.

This issue was discussed on The Firing Line, but nobody knew of any other substantiated event. I have not read of any.

I have never heard of this being a problem in pistols. I imagine it is due to the much faster burn rate of pistol powders.

Loading data is accumulated over time. You don't know if one particular pistol was used for all loads, or that one test with a particular bullet used the same batch or brand of bullet for a different powder--possibly weeks later. And different manuals may well vary with chronographs as well as barrel lengths and primers. Different ambient temperatures affect performance as well. They provide guidelines so you can, as they tell you, start low and work up.

Think of the loading manuals as guidebooks, not Bibles. :)

Art

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