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Thread: BP Load Compression

  1. #1
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    BP Load Compression

    Has anyone done any experimentation on how accuracy, velocity are effected, or if they are effected at all, by the compressing of say 3Fg powder.

    My theory, and itís only a theory mind you, is that the harder a powder is compressed, the slower the burn rate. Two granules of powder, sitting next to one another when the fire travels it will find a path between the two granules and they will burn quickly. However if those two granules are pressed hard together, the fire cannot find a pathway between them and thus travels around them actually slowing the burn rate.

    Does this make sense or am I all wet?

    Chris

  2. #2
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    Your best chance at finding that information from someone in the know is checking the Cowboy Action Shooters for a scientific black powder shooter.

    You may have to look in their monthly published info for that kind of stuff.

    Buckshot

  3. #3
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    My late Uncle did a lot of experimenting on BP including compression. I'll see if I can find his lab notes and scan them for you. The conclusion is that since BP does not 'detonate' i.e. have a detonation zone it depends on surface area for it's burn rate. This is why the relatively large corn of Fg burns slower than 4Fg. Compression, mechanically corns the powder to the surface area of the diameter of the breech and barrel slowing the burn rate accordingly.

  4. #4
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    I was just reading about loads for BPCR long range match shooters using 45-70. They tend to pack the powder in, using a long drop tube and a compression die. They claim some amazingly tiny velocity deviation, which would be important shooting long range with such an arched trajectory. Probably most important, as always, is consistency in loading.

    I've tried to vary the compression in a cap and ball revolver, but the problem is that when the ball starts down into the chamber, it's force, force, force..."Whump!" It's difficult getting a light compression.

  5. #5
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    There was a fellow who found a bunch of original 45-70's in a box from his grand-father (I think...but immaterial here). The primers were gone, but he pulled the bullets and got the powder out. It was no longer granular...just a very large cylindrical 'grain'. He figured it would be neat to light and watch burn. But....it just went BOOM! when the lighter touched it.

    Did the powder change its' properties over the years under the compression in the cartridge? I don't know, but something tells me you'd have to really, really compress black to get the grains so close together as to slow the burning.

    If the lovely Selena would scan the notes here too (please! If you find them) perhaps we all could learn some things about this.

  6. #6
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    Uniformity of compression has been called the most critical factor in loading black powder ammunition (P. Mathews, How-to's for the black Powder Cartridge Rifle Shooter).

    He states that as little as 1/8" less compression has decreased his velocity 40fps.

    Old US 45-70 rounds were highly compressed (when found we used to have to pry the powder out with an awl in order to get a pile to ignite). The 45-70 case hols about 60 grains of 3F powder (uncompressed), and I use 62 grains compressed with a 405 grain bullet. I don't know how the US rounds were loaded to 70 grains of powder under the 500 grian bullet without deforming the case (unless loaded through long drop tubes).

    In non-cartridge rifles most shooters try to get even compression when loading, either by firm pressure on the ramrod or a number of "throws" of the ramrod against the powder charge.

  7. #7
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    I don't know how the US rounds were loaded to 70 grains of powder under the 500 grian bullet without deforming the case (unless loaded through long drop tubes).
    The period issue cases were folded or balloon head construction which holds a bit more powder than our modern solid heads which are more suited to the smokeless high pressure loading they are likely to encounter. So it makes perfect sense that the modern cases won't hold what their great grandfathers would.

  8. #8
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    One of the atrocities my OMB uncle committed in his quest for modern range with obsolete propellents was to actually cut the base off a 45-70 case and use it as bushing inside a wood block. He would then measure a standard weight of BP and grain it with a hydraulic press at various pressures. He would then load the 'grain' in real cartridges and measure the velocities.

    When asked if there is mental illness in the family I am forced to mention him.

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