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Bullet bearing surface

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/03/2016 at 07:02
nralifer View Drop Down
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What is the effect of bullet bearing surface on muzzle velocity and pressure, and how would one go about testing this?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/03/2016 at 09:21
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Ted, you are needed.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/03/2016 at 09:42
3_tens View Drop Down
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If your primers blow pressure is too high. This is why when reloading you work up o the best load.


Per Hodgdon:

Q: What is CUP?

A: Copper Unit of Pressure (CUP) is a measurement used in the ammunition industry to determine the chamber pressure created by a cartridge load. Originally, a precisely formed copper slug was placed in a fixture over the chamber. When the cartridge was fired, the amount of crushing measured on the slug allowed engineers to determine the pressure.

These days, modern electronic transducers provide faster, more accurate measurements of chamber pressures in pounds per square inch (PSI). CUP and PSI are measured to different scales and are NOT interchangeable.

Every barrel would deliver different results for each different bearing surface variant of bullets. This variation almost invalidates a standard testable, then use able result. A sum of variants are part of what determines the Ballistic coefficient assigned to bullets.




Edited by 3_tens - September/03/2016 at 09:54
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/03/2016 at 19:33
nralifer View Drop Down
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Thanks for the replies.  Does any one have experience with the strain gauge type of pressure sensors that can be glued to the chamber?  I would like to use a system like that to test bullets of similar weights that have widely varying amounts of bearing surface.  The idea is to get a better idea of the relationship between bearing surface, breach pressure and muzzle velocity.  As far as I can tell there is little information on this subject. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/22/2016 at 05:59
nralifer View Drop Down
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Recently did an experiment to try and answer the question.  Had 4, 150 gr all copper 30 cal bullets from various manufacturers that differed significantly in the length of their bearing surfaces. Loaded them all in the same brass using the same powder, powder charge, primer and to exactly the same charge density, and shot them from the same rifle.  The only thing that varied from loaded bullet to bullet was the distance to the lands and this was not more than about .020-.030". The result was that the bullet with the lowest bearing surface had the highest muzzle velocity, and the bullet with the most bearing surface had the lowest. The difference between the highest and lowest was 55 fps. The bullets with intermediate length bearing surfaces had muzzle velocities that inversely correlated to their bearing surface lengths.  I do not have pressure testing equipment so I can't say anything about that, but I saw no indication in the brass or primers of excessive pressure.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/22/2016 at 06:15
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I would expect that result, more bearing surface would equate to higher friction between bullet and barrel. All other things being equal I would also expect higher pressures with larger bearing surfaces. JMHO.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/22/2016 at 06:54
nralifer View Drop Down
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I would really like to do pressure testing to see if the lower bearing surface bullet was more efficient (more fps/unit breech pressure)
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/22/2016 at 19:12
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 Too little bullet bearing surface will not stabilize the bullet. An unstable bullet is less accurate. Longer bullets tend to be more accurate and carry more energy farther. Velocity means nothing if the bullet don't hit the target. Just a point to ponder.  Whatever
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/22/2016 at 19:45
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Quite true. The bullet with the least bearing surface that we tested shoots 1/2 MOA out to 450 yds and 2/3 of the bearing surface is in front of the single shank groove with the rest in front of the boat tail and the rear of the groove.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/23/2016 at 08:02
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Agreed, but longer bullets also tend to be heavier, solids not withstanding. You also must spin it faster to stay stable. Why you need a faster twist as the bullet gets longer. A short fat top is more stable at a slower speed than a tall top. That is my best analogy.
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