rotational decay

Discussions of powders, bullets and loading information.

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mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 8:17 pm

I think the key in getting the vents to work correctly Kurt, will be in getting the ports to center in the groove bottoms and not partially into the adjacent land as that will make a lead tearing corner for the bullet to pass over.
Gas ports in black guns work better when so placed, groove center, as far as I know although I have scoped some that were drilled half in half out and it didn't look good to me.
Getting an even partial row of six in the same plane , in the groove bottoms, say on the top three flats will be difficult in an octagon barrel but quite doable in a round. Probably need at least three rows and they will not line up with each other up barrel because of the pitch change but that would not hurt anything. Most ports are cut with EDM because it's hard to clean up a bored hole into a barrel bore.

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 8:21 pm

Bruce I think the first vector escaping gas would take at bullet exit would be lateral no linear and what is viewed in the video's I have seen is gas escapement before the bullet clears the muzzle.

BFD
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Re: rotational decay

Post by BFD » Thu Mar 26, 2020 8:31 pm

escapement?
I see no escapement
https://www.youtube.com/watch?v=TCJnYh9UVMg

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 8:38 pm

One other point I would like to know about happening is base erosion of lead bullets passing over ports in a barrel? I have seen ported .45 acp 1911 barrels and don't remember if they were used with plain base lead bullets successfully.
Mag-na-port used two rows of two trapezoidal vents either side of the front sight and again I don't recall if that was for lead bullet use. The also were EDM cut if I remember correctly.

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 8:49 pm

What's interesting in that shot Brent is look at the gas erupt ahead of the bullet just after it clears about a foot ,just before the patch is expelled. I didn't see any gas at the muzzle ahead of the bullet in that shot sequence either. I think I have a still shot of it happening here some where and will see if I can find it and post.

Kurt
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Re: rotational decay

Post by Kurt » Thu Mar 26, 2020 8:52 pm

Well I've been wanting to find out the effect with the BPCR doing this and nothing will be lost shortening the barrel to a 30" any way.
I will start with a .100" 5 per side and leave room to make them larger if fouling will be a problem, But I would think the paper would protect the bullet shank passing under the holes as long as they aren't too big. I think releasing the pressure behind the bullet before it clears the muzzle might have some plus effect.
If you look at that video and slow it down more, I can do that with the video editor you will see the patch expanding when the bullet base is still in the muzzle. Or you can use a spot action and freeze it as it clears.
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mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 8:55 pm

The burning powder blast vector ,at base clearance ,is lateral in that video and the gas ahead of the bullet just before the patch leaves is translucent or white.

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 9:01 pm

Very exciting Kurt! Can't wait to see what you find out and learn from your experiment!

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 9:55 pm

Another interesting thing about that sequence is that it looks like the patch comes apart from the base to the ogive not the other way around as I had always assumed happened . I had always thought the air from the front of the bullet was peeling it off.

bruce m
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Re: rotational decay

Post by bruce m » Thu Mar 26, 2020 10:07 pm

which raises the question of whether the patch is too thick, too far forward or other things.
I think knowledge of patching might have come a way since that video was made.
we do not know whether the charge was a light one or a heavy one either.
bruce.
ventum est amicus meus

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 11:00 pm

Bruce, I think that gas ahead of the bullet seen in the video, just before the patch comes apart, might be the evacuated, compressed column of bore air, pushed out ahead and then slowing down as the bullet catches up and passes through.

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 11:09 pm

Remember the occasional fatal accidents caused by blanks. That is the column of air pushed out the bore that at close range acts just like a slug. The burning powder is behind the wad but the compressed air column is what punches the hole.

mdeland
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Re: rotational decay

Post by mdeland » Thu Mar 26, 2020 11:23 pm

Another good example of this is done routinely with percussion guns to clear the nipples. The detonating cap alone will make a leave fly up from a long barrel of air being evacuated by the rather small flash of a cap many inches to the rear of the muzzle. The gas from the cap is not what is moving the leaf but rather the evacuated bore air. Now think of this with a charge of powder,wad and bullet behind driving it out at plus mach velocity.

mdeland
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Re: rotational decay

Post by mdeland » Fri Mar 27, 2020 12:39 pm

In Brents video, freeze the frames about six inches out of the barrel. You will notice a donut shape of gas surrounding the bullet,now progress the video and you will see it enlarge then over take the bullet. The translucent colored gas distinguishes it from the burning powder gas which is in a lateral vector and different color from the muzzle. The compressed column of air is coming out of compression and for a short time moving faster than the bullet thus over taking it until the reverse happens.
I believe this is the compressed evacuated bore air being manifested in the translucent coloration by compressed and heated air which is making steam and burning bore oil and/or bullet lube.
I think this is the same manifestation that I spoke of earlier noticing in another slow mo sequence at the muzzle before the bullet emerged.

mdeland
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Re: rotational decay

Post by mdeland » Fri Mar 27, 2020 12:55 pm

I think the compressed evacuated bore air is what is stripping/breaking up the patch from the rear after it is cut and weakened by the land corners on the trip up barrel.

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