What does bullet engraving by rifling tell us?

[Nvreloader]

Dbooksta,

Very interesting and OUTSTANDING photo's,

Questions,

Would not the psi/fps of bullet change the rifling engravement results?

ie, slower fps/psi would show one thing as compared to a higher fps/psi, same rifle/bbl etc.

At approx what was the fps of these bullets, when shot?

Have you tried a lower/higher fps of these bullets, within normal safe psi for your rifle, to see the results of the rifling engagements??.

Food for thought......

Tia,
Don

 

[David Bookstaber]

To first order, no, the muzzle velocity does not change the engraving. Once the bullet is engraved it pretty much tracks the rifling. So the only thing higher pressures and velocities do is increase the rate of spin (proportionately to the muzzle velocity).

I believe I have seen a second order effect, which is a bit of skidding at the beginning of the engraving. I think that if there's a jump and the bullet hits the leade with enough momentum it skids before the rifling fully grabs and engraves it to the barrel's twist rate. This shows up as scuff marks on the front of the bearing surface. I reviewed my samples looking for a clear example of this and couldn't find one, but maybe I'll try to reproduce it next time I'm running a sequence.

These examples were all 300BLK subsonic, so muzzle velocity about 1000fps and estimated peak pressure about 35kpsi.

With peak pressures closer to 60kpsi and a decent jump I bet I can get skid marks.

 

[Canadian bushman]

Full pressure loads generate much more heat along the surface of the bullet. In my experience testing 308 bullets i estimated near 800 deg f. This is subjective to bearing surface and mv.

Not only is the bullet moving faster but it is being forced into the rifling and bore with more force. This scenario is much more likely to create issues like joe salt described with pieces of the bullet galling and being flung off at the beginning of flight, after leaving the barrel.

Depending on the rifling shape, wear and twist the bullets inertia entering the rifling can appear in different ways. It can smear or it can engrave deeper on one side of a land than the other.

The same bullet fired from different barrels can have drastically different results. How do you think the feds can match them with such certainty?

 

[David Bookstaber]

The same bullet fired from different barrels can have drastically different results. How do you think the feds can match them with such certainty?

Perhaps the same way they did hair and bite matching with such certainty? Based on my experience I have grown suspicious of forensic ballisticians that claim to be able to determine based on a recovered bullet that it was fired from a particular sample gun. I would love to do a controlled test to see if they could reliably match even a pristine fired bullet to one barrel from a sample of barrels with the same rifling.

 

[BoydAllen]

A queston:
Do spherical bullets spin drift?

Some time back there was an on line discussion about how the force of a cross wind causes bullets to drift. Folks got wrapped up in the whole turning into (or away from) the wind thing, at which point I pointed out that the most wind sensitive bullets are round balls. This upset one or two. If a theory does not take all situations into account. perhaps we have confused something that happens with a cause. Correlation does not prove causation. Just because two things happen at the same time does not mean that one caused the other. IMO there is a lot of imagining going on here. The important thing is that we can predict what will happen. This may not mean that we fully understand why it happens. If you do not believe this, explain the mechanism by which gravity works.

 

[Canadian bushman]

A queston:
Do spherical bullets spin drift?

Some time back there was an on line discussion about how the force of a cross wind causes bullets to drift. Folks got wrapped up in the whole turning into (or away from) the wind thing, at which point I pointed out that the most wind sensitive bullets are round balls. This upset one or two. If a theory does not take all situations into account. perhaps we have confused something that happens with a cause. Correlation does not prove causation. Just because two things happen at the same time does not mean that one caused the other. IMO there is a lot of imagining going on here. The important thing is that we can predict what will happen. This may not mean that we fully understand why it happens. If you do not believe this, explain the mechanism by which gravity works.

I believe the most common round ball shooting is musket fire, or bb whice both are smoothbore?
This bullet should not experience spin drift (gyroscopic drift) because it is not spin stabilized. At least not to a predictable amount.
These bullets do drift a lot in the wind because they have very high drag from their 1/2 caliber radius, they also slow down quickly.
From my experience casting and with round shot, very few of them are actually round. Most are all dented or have a flat spot especially in particular ones that have a tangenial sprue plate. I could only imagine what a semi round ball with a large flat spot is gonna do when it faces a 700mph head wind.

Unlike rifling which causes scarring equally around the entire bullet, which acts to offset each others wind drag from steering the bullet, flats and dents on a musket ball are never symetrical, so i imagine they would probably shoot very unpredictably. Just like bb's.

This is also why the minie ball was a huge leap forward in musket fire.

 

[David Bookstaber]

A queston:
Do spherical bullets spin drift?

Theoretically, if fired from a rifled barrel, they should, regardless of crosswind: They still follow a ballistic trajectory, so they do have a positive yaw of repose (i.e., the spin axis is elevated relative to the direction of travel).

 

[Canadian bushman]

A round bullet cannot tip in to the direction of its spin causing it to spin drift.

 

[David Bookstaber]

A round bullet cannot tip in to the direction of its spin causing it to spin drift.

Because they have no "tip?" Semantically true, but they do have a spin axis, and they will spin drift. If you watch baseball you can see pitchers produce the same effect with round balls, and the physics are the same.

 

[Canadian bushman]

A round bullet cannot tip in to the direction of its spin causing it to spin drift.

Because they have no "tip?" Semantically true, but they do have a spin axis, and they will spin drift. If you watch baseball you can see pitchers produce the same effect with round balls, and the physics are the same.

Its actually scientifically true, not semantics. Believe it or not, agree with it or not, round balls actually do not have a tip.

They have no tip.
They dont benefit from being stabilized since everyside of the geometry is the same. Typically why muskets that fire round balls dont have rifling.

The pitcher creates effects of the "round ball" (with laces) by changing the direction of the axis of rotation in relation to the direction of travel. Which is why they hide their hands from the batter. Although absolutely governed by physics, it is a completely different scenario with entirely different physics.

IF you have a perfectly round musket ball, and IF you fired it from a rifled barrel, the chances the axis of rotation will align itself in any predictable manner to the direction of forward movement is highly unlikely.

IF it did align itself, and IF it maintained that alignment, it would not experience spin drift because of the geometry of the round bullet. Assuming you could get the musket ball to even travel down the barrel with its axis of rotation parellel to the axis of the bore. A whole lotta IF's in a completely un-comparible scenario to disprove a function of modern day rifles that has already been prooven, and recently comfirmed by a masterful ballistician.

As much as i enjoy debating the possible outcomes of completely impractical scenarios of no practical relevance to modern long range shooting. There is meaningfull work that needs to be done, and unfortunately im the only one who is gonna do it.

 

[dcali]

A round bullet cannot tip in to the direction of its spin causing it to spin drift.

Because they have no "tip?" Semantically true, but they do have a spin axis, and they will spin drift. If you watch baseball you can see pitchers produce the same effect with round balls, and the physics are the same.

Baseballs don't drift due to spin drift. Baseballs curve because of a different phenomenon that has to do with something called the magnus force. This doesn't happen with bullets to a significant degree.

Spin drift is a very specific thing: it is due to the axis of symmetry of the bullet being tipped to the side by a combination of gravity and the bullet's spin (the yaw of repose). The air flow, now being asymmetrical, results in a sideways lift force which causes the drift. Perfectly round bullets cannot be impacted by spin drift because they cannot orient in a manner that is asymmetrical to the air flow.

 

[David Bookstaber]

So round bullets shot from a right-hand rifled barrel drift to the right due entirely to the magnus effect?

As I posted earlier, my understanding of "spin drift" (i.e., the tendency for a bullet to drift rightward from its initial trajectory, and to do so proportionally to its spin rate) referred to a combination of magnus and gyroscopic effects created when a bullet is pulled downward by gravity. I thought we could discriminate the two because the gyroscopic effects are independent of the atmosphere, but the magnus effect only occurs in an atmosphere (and increases with air density).

Based on this understanding the gyroscopic spin drift still occurs in round bullets because they experience a downward force due to gravity, which results in positive (upward) yaw of repose. Airflow is irrelevant to that effect.

Of course I'm always learning, so please let me know where I'm mistaken.

 

[dcali]

I don't shoot round balls, but if they experience spin drift, it's because they aren't perfectly round. Perhaps being forced through the rifling is enough to give them enough shape to be subject to the effect. I don't really know. I have always assumed that round balls shoot better out of rifled barrels because they permit a better bore/bullet fit, and when you do that, you no longer have a truly round bullet, so a little twist is helpful in maintaining a stable orientation. But I honestly don't know exactly why round balls shoot better out of rifled barrels, as I haven't spent much time thinking about it.

The magnus force is insignificant in bullets (for the purposes of this discussion, at least), so we don't really see the effect. But even if it were significant, it would require yaw (if the airflow is in line with the spin axis, the magnus force is zero). It would also point in the wrong direction (up/down, rather than left right). So that's not it.

The lift force is entirely dependent on aerodynamic asymmetry, which cannot exists unless the bullet is asymmetrical itself.

 

[David Bookstaber]

The magnus force is insignificant in bullets (for the purposes of this discussion, at least), so we don't really see the effect. But even if it were significant, it would require yaw (if the airflow is in line with the spin axis, the magnus force is zero). It would also point in the wrong direction (up/down, rather than left right). So that's not it.

Typically when ballisticians talk about the magnus force they are referring to a lifting force (i.e., upward or downward resulting from a crosswind). I thought this was because for purposes of lateral drift they have rolled it together with the gyroscopic force and just called the cumulative effect "spin drift." I was thinking of the magnus force caused by the bullet falling under gravity, although looking at it carefully now it appears that should be a left-ward force for a right-ward spin, so I guess that's not the right term.

So for the master ballisticians here's the question: Does the gyroscopic effect entirely explain lateral drift of small rifle bullets, excluding crosswind? If so how does the air density figure into the equation? (I have my McCoy open to section 10.6!)

And back to the original questions raised in this thread: Does friction against the sides of a spinning bullet have any effect? It must change the scale of the magnus force. But if the deformations that even the harshest rifling introduces don't produce any other effect (e.g., like the moments of a finned projectile I suggested earlier) then I guess this is at most third-order for external ballistics, and we need not think about it so long as the bullet holds together!

 

[dcali]

You have to forget about the idea of wind blowing on a bullet or falling through the air. There is only airflow relative to the bullet. It is almost completely moving opposite the motion of the bullet.

If the bullet is pointing straight into that oncoming air flow (for example, if the nose is perfectly tracking the trajectory) without yaw, the only impact the air will have is to produce a drag force. There bullet is symmetrical axially, so there is no bullet geometry that will result in a sideways force. The spin axis is perfectly aligned with the air flow, so the magnus force is also zero.

All that changes a little if the bullet yaws. It tips sideways relative to the trajectory, exposing the side of the bullet to the oncoming air flow. The bullet is no longer symmetrical in it's cross section to the trajectory, and you get a sideways force as the air tends to push the bullet in the direction its nose is tipped. This is called the "lift force", which is poorly named, because it can be in any direction, not just up, as the name might imply.

Spin drift is just lift generated by the yaw of repose (the average yaw as the bullet flies), which is created by a gravitational effect. Gravity wants to tip the bullet nose up, but because it's spinning, it winds up pointing in the direction of the spin, much the way a spinning bicycle wheel wants to rotate 90 degrees from the way you try to push it.

Yaw will also expose the side of the bullet to the airflow, which will create a situation where there is a non-zero magnus force. A sideways yaw will result in a vertical magnus force. But as I said, it's a very small force and typically neglected by ballisticians, except as it relates to the magnus *moment* (the rotational force caused by the linear magnus force) which is important for things like stability.

 

[XTR]

A round bullet cannot tip in to the direction of its spin causing it to spin drift.

Because they have no "tip?" Semantically true, but they do have a spin axis, and they will spin drift. If you watch baseball you can see pitchers produce the same effect with round balls, and the physics are the same.

Its actually scientifically true, not semantics. Believe it or not, agree with it or not, round balls actually do not have a tip.

They have no tip.
They dont benefit from being stabilized since everyside of the geometry is the same. Typically why muskets that fire round balls dont have rifling.

There were and are a whole lot of rifled muzzle loaders that shot/shoot round balls going back to the 18th century. Whether or not the weapons of the past era were rifled, the muzzle loaders used today are, and they can and do shoot stabilized round balls from barrels with in the neighborhood of a 1:22 twist.

 

[David Bookstaber]

These examples were all 300BLK subsonic, so muzzle velocity about 1000fps and estimated peak pressure about 35kpsi.

With peak pressures closer to 60kpsi and a decent jump I bet I can get skid marks.

@Nvreloader: Here's a comparison of the original low-pressure load (bottom) and one with a faster powder (top) that took peak pressure to about 60kpsi (estimated by QuickLOAD and suggested by primer flattening but no other pressure signs). You can see two "skid marks" just above and slightly in front of the final engraving groove.

Interestingly, the secondary "groove" marks are mostly absent on the high-pressure bullet. One wild guess: the higher-pressure engraving cuts into the jacket more, reducing the deformation pressure that in the lower-pressure bullet is relieved against the groove opposite each land.



For reference: Both of these are 220gr SMKs from the same 12" barrel. Muzzle velocity of the low-pressure load is about 1000fps, and this high-pressure load produced 1225fps.