Does a moly bullet coating affect BC ?

[big5ifty]

Has anyone noticed that their moly coated bullet does not drop as much as the ballistic calculator predicts ?

Referring specifically to results with the Lyman shot-peening method.

This is attributed to Dan Hacket, but I can't find anything else.

"As the only person I know of who has published articles about trajectories becoming flatter with moly-coated bullets, I think I can say I know something about the topic. First, I do not merely claim a reduction in bullet drop; I report it as a matter of objective fact. This is a real phenomenon. It is not something I thought I noticed while shooting offhand at a rock on the far side of some canyon. It is an occurrence I have repeatedly observed, measured, verified, duplicated and recorded with at least as much care as Mr. Vaughn reports using anywhere in his book, including tests I have performed with 6mm benchrest bullets similar to the ones he employed."

 

[SDDasher]

Has anyone noticed that their moly coated bullet does not drop as much as the ballistic calculator predicts ?

Referring specifically to results with the Lyman shot-peening method.

This is attributed to Dan Hacket, but I can't find anything else.

"As the only person I know of who has published articles about trajectories becoming flatter with moly-coated bullets, I think I can say I know something about the topic. First, I do not merely claim a reduction in bullet drop; I report it as a matter of objective fact. This is a real phenomenon. It is not something I thought I noticed while shooting offhand at a rock on the far side of some canyon. It is an occurrence I have repeatedly observed, measured, verified, duplicated and recorded with at least as much care as Mr. Vaughn reports using anywhere in his book, including tests I have performed with 6mm benchrest bullets similar to the ones he employed."

I concur. I typically have to use less elevation than what the ballistic calculators recommend with MC Bullets.

 

[Bill Norris]

Interesting info.

 

[dcali]

Back in the day, people used to swear that it does. I've never shot them myself, but that doesn't keep me from thinking they were mistaken. There is nothing in the physics of ballistics that would suggest that the coating on a bullet matters at all. That doesn't' mean that there couldn't be some sort of in bore effect that changes the launch, but it's not a matter of the surface being more "slippery" in the air.

 

[big5ifty]

I concur. I typically have to use less elevation than what the ballistic calculators recommend with MC Bullets.

This weekend I was at 600 meters, holding 8.7 moa instead of 10.7, with the 4 and 500 also incrementally different, and impacting consistently.

Today I was playing with the numbers in JBM trying to make sense of it, and I got the drop to line up on the three distances by increasing the BC quite drastically.

 

[drop_point]

I used to shoot a lot of moly. Only thing I ever found was that it was about 25 fps less with it than without it. Never saw any less drop. Don't see how a coating could affect BC.

 

[Charlie-NY]

I shot a zillion moly-coated bullets at the range and in the groundhog fields. Even my 8" twist 22-243AI never dusted a bullet. My thinking is that the moly helps smooth the engraving process and causes less stress on thin jackets. If the moly leads to a smoother bullet exterior after engraving it might preserve higher BC.

Just guess work on my part.

 

[Trevor60]

Based on a thread over on benchrest central it appears that moly bullets have less deformation than non coated bullets... see link - Posted by coyotechet
This support Hackets information that Moly'd bullets (retain) more of their original BC than non coated bullets.
I don't have any comparable data as i have always shot moly'd bullets but for other reason.

Trevor

 

[gunsandgunsmithing]

I shot a zillion moly-coated bullets at the range and in the groundhog fields. Even my 8" twist 22-243AI never dusted a bullet. My thinking is that the moly helps smooth the engraving process and causes less stress on thin jackets. If the moly leads to a smoother bullet exterior after engraving it might preserve higher BC.

Just guess work on my part.

I think it's a long shot that it matters but that's at least plausible and never entered my feeble mind.

 

[Ned Ludd]

Back in the day, people used to swear that it does. I've never shot them myself, but that doesn't keep me from thinking they were mistaken. There is nothing in the physics of ballistics that would suggest that the coating on a bullet matters at all. That doesn't' mean that there couldn't be some sort of in bore effect that changes the launch, but it's not a matter of the surface being more "slippery" in the air.

Matter is matter, a surface is a surface, and an interaction between the two is an interaction. Why would a smoother surface not have an effect on BC? If the moly coating reduces bullet friction against the bore, why would it not do so in the air? I would certainly guess that roughing up the surface of a bullet nose with very coarse sandpaper would be detrimental to BC, although I have never tried it. Is what you're really saying that there IS an effect, but it's simply so small as to be negligble? Unlike the moly-metal interface friction in the bore, I'm guessing the bullet jacket-air interface friction is already so small that the effect of changing it slightly with moly just isn't detectable by most of the methods we would typically use (i.e. velocity, drop, etc.).

 

[gunsandgunsmithing]

Friction and drag are two different things. If moly made a bullet slicker to air, I think fighter jets would be coated with it. Either way, if the difference is so negligible it's not seen, it's not worth arguing over either. But that my 2 cents.

 

[6fatrat]

Interesting thought.
I may try an experiment this summer at one of our 1000yd matches.
Steve Bair

 

[big5ifty]

I may try an experiment this summer at one of our 1000yd matches.

Not sure what moly method you use, but I recommend the Lyman method, which is dry, no wax, and uses ceramic media. The instructions can be downloaded from Lyman.

This is a shot-peening method, and if you use a metal insert in a tumbler, the results are better than if it's plastic.

I found that replacing the plastic bowl of a tumbler with a steel bundt cake mould and a makeshift lid works very well.

When done right, the moly does - correction : not - rub off easily. Handling the moly coated bullets will not leave your fingers black.

It took me a while to figure out what people meant when they said that moly bullets made a mess with build-up in their barrels. They probably flirted with the wax method.

There is a method that coats the bullet with moly and wax, which to my mind is equivalent to tumbling the bullets in automotive grease, and completely defeats the purpose of applying moly in the first place.

 

[ballisticdaddy]

Just did some basic math, for you to have 2 MOA less drop the bullet would have to be traveling +/- 300 fps and by adjusting the BC alone was not able to duplicate those drops. If your numbers are correct then I have no idea why EVERY bullet manufacturer would not moly coat their bullets? A few years back while shooting FT-R I dabbled with moly coating and did not see anywhere near your results.

 

[dcali]

Matter is matter, a surface is a surface, and an interaction between the two is an interaction. Why would a smoother surface not have an effect on BC? If the moly coating reduces bullet friction against the bore, why would it not do so in the air? I would certainly guess that roughing up the surface of a bullet nose with very coarse sandpaper would be detrimental to BC, although I have never tried it. Is what you're really saying that there IS an effect, but it's simply so small as to be negligble? Unlike the moly-metal interface friction in the bore, I'm guessing the bullet jacket-air interface friction is already so small that the effect of changing it slightly with moly just isn't detectable by most of the methods we would typically use (i.e. velocity, drop, etc.).

The idea is that there is a boundary layer between the surface of the bullet and the air moving around it. I know it sounds weird, and like a theoretical fudge factor, but the air in contact with the bullet is not really moving at all. And by in contact, I mean at a molecular level. Shear forces in the air gradually accelerate the air as it moves out away from the bullet until it's far enough away to not be impacted by the bullet at all. Theoretically, if you assume there is zero movement between the surface and the air, that's a "no slip condition". And in reality, that is strangely (to me, at least) turns out to be a fairly good assumption.

A very rough surface is huge in comparison to the few molecules at the surface of the bullet. I don't know exactly what that would do. Somewhere between nothing to possibly inducing a turbulent boundary layer. Might be fun to figure out.

I don't know the extent to which the air actually moves over a bullet, or if moly changes that at all, but it strikes me as exceedingly unlikely to be anything approaching significant.

What does matter is skin friction drag, but it's not really friction in the sense that we normally think of it (say, two pieces of metal sliding against each other). It's the result of the viscosity of the boundary layer dragging the molecules of air that are stuck to the surface of the bullet. It's a function of the surface area and the characteristics of the flow. There is a significant difference in skin friction drag depending on whether or not the boundary layer flow is turbulent or laminar, for example.

Long story short, I would be very surprised if there was a measurable impact on drag. I've been wrong before and will be many times again, though.

Side note on moly - Harold Vaughn believed that the measurable impact of moly on velocity was due to the heat required to vaporize the moly, and not because of any changes in friction. I don't know if he was right, but he made a pretty good case. He found that moly coating bullets reduced pressure and velocity. He also tried alternating coated and uncoated bullets, and found that it did not change anything. Coated bullets still had lower pressure and velocity than uncoated bullets. So it's not a matter of moly coating the bore. But what's more interesting is that he tried putting moly *inside* the case and found it had the same effect as putting it on the bullet.

If he's right, then a lot of the moly will be gone by the time the bullet exits the barrel.

 

[Ned Ludd]

The idea is that there is a boundary layer between the surface of the bullet and the air moving around it. I know it sounds weird, and like a theoretical fudge factor, but the air in contact with the bullet is not really moving at all. And by in contact, I mean at a molecular level. Shear forces in the air gradually accelerate the air as it moves out away from the bullet until it's far enough away to not be impacted by the bullet at all. Theoretically, if you assume there is zero movement between the surface and the air, that's a "no slip condition". And in reality, that is strangely (to me, at least) turns out to be a fairly good assumption.

A very rough surface is huge in comparison to the few molecules at the surface of the bullet. I don't know exactly what that would do. Somewhere between nothing to possibly inducing a turbulent boundary layer. Might be fun to figure out.

I don't know the extent to which the air actually moves over a bullet, or if moly changes that at all, but it strikes me as exceedingly unlikely to be anything approaching significant.

What does matter is skin friction drag, but it's not really friction in the sense that we normally think of it (say, two pieces of metal sliding against each other). It's the result of the viscosity of the boundary layer dragging the molecules of air that are stuck to the surface of the bullet. It's a function of the surface area and the characteristics of the flow. There is a significant difference in skin friction drag depending on whether or not the boundary layer flow is turbulent or laminar, for example.

Long story short, I would be very surprised if there was a measurable impact on drag. I've been wrong before and will be many times again, though.

Side note on moly - Harold Vaughn believed that the measurable impact of moly on velocity was due to the heat required to vaporize the moly, and not because of any changes in friction. I don't know if he was right, but he made a pretty good case. He found that moly coating bullets reduced pressure and velocity. He also tried alternating coated and uncoated bullets, and found that it did not change anything. Coated bullets still had lower pressure and velocity than uncoated bullets. So it's not a matter of moly coating the bore. But what's more interesting is that he tried putting moly *inside* the case and found it had the same effect as putting it on the bullet.

If he's right, then a lot of the moly will be gone by the time the bullet exits the barrel.

I see. It's analogous to the "bubble" of subspace surrounding the U.S.S Enterprise while at warp.

Thanks for the explanation Damon, I appreciate it!

 

[big5ifty]

... I dabbled with moly coating and did not see anywhere near your results.

Out of interest, how did you apply the coating ?

 

[dgeesaman]

The idea is that there is a boundary layer between the surface of the bullet and the air moving around it. I know it sounds weird, and like a theoretical fudge factor, but the air in contact with the bullet is not really moving at all. And by in contact, I mean at a molecular level. Shear forces in the air gradually accelerate the air as it moves out away from the bullet until it's far enough away to not be impacted by the bullet at all. Theoretically, if you assume there is zero movement between the surface and the air, that's a "no slip condition". And in reality, that is strangely (to me, at least) turns out to be a fairly good assumption.

A very rough surface is huge in comparison to the few molecules at the surface of the bullet. I don't know exactly what that would do. Somewhere between nothing to possibly inducing a turbulent boundary layer. Might be fun to figure out.

I don't know the extent to which the air actually moves over a bullet, or if moly changes that at all, but it strikes me as exceedingly unlikely to be anything approaching significant.

What does matter is skin friction drag, but it's not really friction in the sense that we normally think of it (say, two pieces of metal sliding against each other). It's the result of the viscosity of the boundary layer dragging the molecules of air that are stuck to the surface of the bullet. It's a function of the surface area and the characteristics of the flow. There is a significant difference in skin friction drag depending on whether or not the boundary layer flow is turbulent or laminar, for example.

Long story short, I would be very surprised if there was a measurable impact on drag. I've been wrong before and will be many times again, though.

Side note on moly - Harold Vaughn believed that the measurable impact of moly on velocity was due to the heat required to vaporize the moly, and not because of any changes in friction. I don't know if he was right, but he made a pretty good case. He found that moly coating bullets reduced pressure and velocity. He also tried alternating coated and uncoated bullets, and found that it did not change anything. Coated bullets still had lower pressure and velocity than uncoated bullets. So it's not a matter of moly coating the bore. But what's more interesting is that he tried putting moly *inside* the case and found it had the same effect as putting it on the bullet.

If he's right, then a lot of the moly will be gone by the time the bullet exits the barrel.

Interesting stuff.

Yeah, it's not intuitive, but any particle in a fluid has to move based on the particles around it. So for any body moving in a fluid, the fluid particles against the moving body pretty much move with it, while particles slightly further away are moving but not as fast, and so on.

So the proposal is that moly coating on the bullet does not cause higher velocity, but the moly coated bullet has less drop. That means it improved ballistic coefficient. Seems like something Brian Litz should have tested, but I'm not seeing it mentioned in any of his books. He does talk about meplat treatments. If the effect was that profound I'm sure all of the 1000y benchrest shooters would be doing it though. Worthy of more research.

David

 

[dcali]

Interesting stuff.

Yeah, it's not intuitive, but any particle in a fluid has to move based on the particles around it. So for any body moving in a fluid, the fluid particles against the moving body pretty much move with it, while particles slightly further away are moving but not as fast, and so on.

So the proposal is that moly coating on the bullet does not cause higher velocity, but the moly coated bullet has less drop. That means it improved ballistic coefficient. Seems like something Brian Litz should have tested, but I'm not seeing it mentioned in any of his books. He does talk about meplat treatments. If the effect was that profound I'm sure all of the 1000y benchrest shooters would be doing it though. Worthy of more research.

David

I really doubt there's any effect at all. It's a very simple test to do, and the major bullet manufacturers have the radars needed to do it. Heck, even an Ohler system would do it.

 

[big5ifty]

Just did some basic math, for you to have 2 MOA less drop the bullet would have to be traveling +/- 300 fps and by adjusting the BC alone was not able to duplicate those drops.

According to JBM, my velocity has to go from 3150 to 3250 to roughly match the observed drop.

Or the G1 went to around .68. Or something somewhere is very wrong.

.308 Berger 168.