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Tumbled bullets = poor accuracy?

 
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Old 05-19-2011, 01:01 PM
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Tumbled bullets = poor accuracy?

A few years back I had a custom 338 Edge built on a 700 by a reputable smith. The gun never shot very well. Usually right around 1 MOA. I tried just about every load I could think of with the 300 SMK but it never got better. After about 250 rounds, I sold it. I explained to the buyer the issues and he seemed not to care. Just before I sold it, I took it out for one more trial. It shot two three-shot clover leaf groups at 125 yards with loads of 91 and 92 grains of H-1000. I know the current owner and he is as pleased as can be with the gun. He claims 0.5 MOA to as far as he cares to shoot.

This got me thinking…and kicking myself for selling it. The bullets I had been firing were purchased with moly coating. I wanted nothing to do with the moly so at the recommendation of a few others, I tumbled it off. After looking at the cross section pictures of the 300 SMK that Shawn Carlock posted, I wonder if the cavity filled with tumbling media causing poor accuracy somehow? The two cloverleaf groups were with new bullets that had not been tumbled. I just chalked those groups up to a fluke.

Any thoughts on this?

Thanks.
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Old 05-19-2011, 02:43 PM
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Re: Tumbled bullets = poor accuracy?

If a frontal cavity had tumbling media in it, it would definitely alter the bullet’s internal and external ballistics.

On a secondary note, there is a reason why you hardly hear of reloaders tumbling their bullets. When you tumbled off the moly of your bullets, you also reduced the diameter of your bullets. This, of course, would provide a ‘loose fit’ in your bore. In addition, when you tumble bullets, brass, etc…they become polished and shiny because you have removed a layer of the surface. This removal is not exact regarding circumference dimensions and will lead to the bullet entering the bore on a slight misalignment upon ignition. To make matters worse, now that the circumference is uneven, you have shifted the bullets center of gravity away from its central axis, adding an additional ‘wobble’ to the wobble of the misaligned bullet entering the bore.

To answer your heading question: Tumbled bullets = poor accuracy? You betcha!
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Old 05-20-2011, 09:26 AM
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Re: Tumbled bullets = poor accuracy?

No, the media in the nose won't cause an accuracy loss. To begin with, it has very little mass, and when it's in the nose it's located very close to the centerline (and along the axis of the CG) of the bullet. This minimizes any effect that it might exert if it were located further out, say closer to the jacket wall at the bearing surface. Saw this quite frequently before Sierra switched to a larger grade of nutshell that wouldn't pass through the hollowpoints of MK bullets. Prior to that, they had used a very fine grade of nutshell which would fill the nose. Never caused any accuracy problems, but there were lots of customer complaints about opening up a new box of bullets and finding nutshell in the bottom which had come out of the noses during shipment. So the polishing is strictly cosmetic, and done with the knowledge that it does degrade accuracy a bit. That frontal portion of the bullet is a surprisingly insensitvie area. Honestly, most shooters would be astonished to see just how badly you can bugger up the nose of a bullet without having any deleterious effect on accuracy, at least at shorter ranges (100, maybe even 200 yards or so). So no, in the pile of potential problems in bullet construction, a little media in the nose cavity doesn't amount to much of anything.

The tumbling process itself is a different matter, and yes, it degrades accuracy. The most accurate that bullet will ever be, is when it's punched out the sizing die and into the collection box below. From that point on, any step that involves handling the bullet increases the likelyhood that it will be damaged and the accuracy degraded. Most shooters like the nice, shiney finish of a new box of bullets. Most competitive shooters, however, have been after them for many, many years now to package the bullets untumbled, right as they come off the machine. In tumbling them to remove the oil (originally, at the factory) and then again to apply the moly, and finally a third time to attempt to remove the moly, yeah, there's a lot of opportunities to degrade the accuracy of the bullet during all that rock 'n rolling. My guess would be, that's exactly what happened.
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Old 05-21-2011, 03:42 AM
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Re: Tumbled bullets = poor accuracy?

Quote:
Originally Posted by Kevin Thomas View Post
No, the media in the nose won't cause an accuracy loss. To begin with, it has very little mass, and when it's in the nose it's located very close to the centerline (and along the axis of the CG) of the bullet. This minimizes any effect that it might exert if it were located further out, say closer to the jacket wall at the bearing surface. Saw this quite frequently before Sierra switched to a larger grade of nutshell that wouldn't pass through the hollowpoints of MK bullets. Prior to that, they had used a very fine grade of nutshell which would fill the nose. Never caused any accuracy problems, but there were lots of customer complaints about opening up a new box of bullets and finding nutshell in the bottom which had come out of the noses during shipment. So the polishing is strictly cosmetic, and done with the knowledge that it does degrade accuracy a bit. That frontal portion of the bullet is a surprisingly insensitvie area. Honestly, most shooters would be astonished to see just how badly you can bugger up the nose of a bullet without having any deleterious effect on accuracy, at least at shorter ranges (100, maybe even 200 yards or so). So no, in the pile of potential problems in bullet construction, a little media in the nose cavity doesn't amount to much of anything.

The tumbling process itself is a different matter, and yes, it degrades accuracy. The most accurate that bullet will ever be, is when it's punched out the sizing die and into the collection box below. From that point on, any step that involves handling the bullet increases the likelyhood that it will be damaged and the accuracy degraded. Most shooters like the nice, shiney finish of a new box of bullets. Most competitive shooters, however, have been after them for many, many years now to package the bullets untumbled, right as they come off the machine. In tumbling them to remove the oil (originally, at the factory) and then again to apply the moly, and finally a third time to attempt to remove the moly, yeah, there's a lot of opportunities to degrade the accuracy of the bullet during all that rock 'n rolling. My guess would be, that's exactly what happened.
Kevin you seem to be discounting something here which seems so obvious to me it would be easy to overlook.

If tumbling media were buried in the cavity I agree it would probably have very little effect on accuracy, but if the slightest bit protrudes from that cavity it could/would I think be capable of completely changing the flight characteristics of the bullet to the same extent a bent tip on a sold nose or mauled tip on a soft point could have.

Seems to me the likelihood of a destabilization right out of the bore would likely cause a considerable yaw problem in the first few feet out of the bore.bent tip.

That effect would moderate rapidly as that tiny bit of protruding media flaked off, but the damage done in the first few feet to the line of flight I would think would be very likely to make it all but impossible to shoot decent groups.

That such a problem would not be consistent with all bullets would then make the problem even worse. If two or three out of a five shot group were "fliers" for that very reason, it would blow the whole group pretty bad.... .

As for the rest it sounds spot on.
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Old 05-23-2011, 08:22 AM
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Re: Tumbled bullets = poor accuracy?

It would seem that way, and it's a logical assumption. But that's not the case. Shooters tend to be very focused on the nose of bullets, the meplats and forward surfaces of the ogives directly behind there, but it's in truth probably the most insensitve portion of the bullet where flight dynamics are concerned. Just to be clear, however, I should specify that I'm talking about short range accuracy here. There can and will be changes in BC when the noses aren't uniform, and that will in turn affect vertical dispersion at greater distances. Varying BC's also make windage holds/adjustment more of a wild-assed guess than an exercise in true doping wind. So from that standpoint, yeah, I guess you could say accuracy is impacted.

As far as the pieces of media protruding and becoming dislodged during flight, no, I can't go along with that one. To begin with, they'd never withstand either the forces of the launch, or the heat generated during flight. As to the changing of shape during that flight, again, it's not going to be a factor in any meaningful sense. Lead tips on spitzers, for example, alter their shape during flight as surface areas reach melting point and begin to be peeled away by atmospheric resistance. So long as they do so on a fairly consistent basis from shot to shot, they're still capable of shooting decent groups even at longer ranges. On a larger scale, some types of ICBM re-entry vehiciles actually use this process (it's known as ablation) as a cooling method to shed excess heat. So, some degree of shape change, so long as it remains consistant from shot to shot, just won't make that much difference.

The other point that needs to be considered is where theis media would be located as you view the bullet head on. I tried to make that clear in my initial post, but perhaps I wasn't able to get my point across. Anyway, here goes again. When you look at the bullet from the nose on, the defects at the nose (be they media in the hollowpoint or some other deformity), are actually located very close to the center of mass of the bullet as it's in flight. Think of it this way; we tie a weight to a very short string of just an inch or so and swing it in a circle. Then we tie the same weight to another string three feet long and swing that in a circle as well. The centrifugal forces acting on the longer arc are much more apparent. It's the same when the variance (whatever imbalance we're talking about here) is located very close to the centerline of the axis of a bullet, as compared to when it's located out towards the jacket. . An air void located precisely in the center of that axis would have no effect on the balance of that bullet, whereas if it's loacated out against the jacket wall, you have a major problem. I hope that's a bit clearer.
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Last edited by Kevin Thomas; 05-23-2011 at 11:00 AM.
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Old 05-23-2011, 11:14 PM
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Re: Tumbled bullets = poor accuracy?

Quote:
Originally Posted by Kevin Thomas View Post
It would seem that way, and it's a logical assumption. But that's not the case. Shooters tend to be very focused on the nose of bullets, the meplats and forward surfaces of the ogives directly behind there, but it's in truth probably the most insensitve portion of the bullet where flight dynamics are concerned. Just to be clear, however, I should specify that I'm talking about short range accuracy here. There can and will be changes in BC when the noses aren't uniform, and that will in turn affect vertical dispersion at greater distances. Varying BC's also make windage holds/adjustment more of a wild-assed guess than an exercise in true doping wind. So from that standpoint, yeah, I guess you could say accuracy is impacted.

As far as the pieces of media protruding and becoming dislodged during flight, no, I can't go along with that one. To begin with, they'd never withstand either the forces of the launch, or the heat generated during flight. As to the changing of shape during that flight, again, it's not going to be a factor in any meaningful sense. Lead tips on spitzers, for example, alter their shape during flight as surface areas reach melting point and begin to be peeled away by atmospheric resistance. So long as they do so on a fairly consistent basis from shot to shot, they're still capable of shooting decent groups even at longer ranges. On a larger scale, some types of ICBM re-entry vehiciles actually use this process (it's known as ablation) as a cooling method to shed excess heat. So, some degree of shape change, so long as it remains consistant from shot to shot, just won't make that much difference.

The other point that needs to be considered is where theis media would be located as you view the bullet head on. I tried to make that clear in my initial post, but perhaps I wasn't able to get my point across. Anyway, here goes again. When you look at the bullet from the nose on, the defects at the nose (be they media in the hollowpoint or some other deformity), are actually located very close to the center of mass of the bullet as it's in flight. Think of it this way; we tie a weight to a very short string of just an inch or so and swing it in a circle. Then we tie the same weight to another string three feet long and swing that in a circle as well. The centrifugal forces acting on the longer arc are much more apparent. It's the same when the variance (whatever imbalance we're talking about here) is located very close to the centerline of the axis of a bullet, as compared to when it's located out towards the jacket. . An air void located precisely in the center of that axis would have no effect on the balance of that bullet, whereas if it's loacated out against the jacket wall, you have a major problem. I hope that's a bit clearer.
Kevin you make two points there that make perfect sens.

1) I hadn't considered the fact that most of the tumbling media we use are highly combustable such as corn cobb or walnut shells. They would certainly disintegrate and fly off as ash quickly with any high speed round.

2) On the sampe point, what I was thinking of would be the media not having a smooth but rough point which would be off center would change the flight characteristics considerably, but then, since it will have burned off almost instantly, that issue is gone.

Good points.

I have actually fired some FMJ's over the years with a bent tip which were completely useless past about 100yds which is what got me to thinking along those lines.

The two are of course in no way related because of your points above.

Thanks. WR
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