What does bullet engraving by rifling tell us?

[David Bookstaber]

If the rifling were rounded, how would the bullet seal the gases, and engrave the jacket to start the spinning process? In addition to imparting spin to the bullet, the barrel must hold the bullet securely and concentrically as it travels through the barrel. I can't see that occurring with rounded rifling.

Perhaps I didn't explain what I meant by "rounding" well enough. A little searching shows this has and is widely used, and is generically called "polygonal" rifling.

I guess the drawback is as well known as the problem with shooting lead bullets in Glock barrels: With no "slop" in the rifling corners for a slightly oversized bullet (or, equivalently, a regular bullet in a bore shrunken by fouling) to deform, or for overpressure gas blow-by, any overbore bullet results in higher barrel pressures.

Now, for precision shooters, it seems like one would much rather have less deformation of one's bullets and the perfect gas seal offered by polygonal rifling, at the expense of maintaining a clean bore, so I'm still wondering why this isn't the most common rifling for precision barrels.

 

[XTR]

Here is my thought, what you are seeing is that the bullet is not conforming exactly to the shape of the barrel next to the lands, it is being bent in and creating a bit of a radius (which is why after you've cleaned the heck of your barrel you still get black streaks on a tight patch at the edges of the grooves) the streak on the bullet that shows up between the marks of the lands is where the jacket is actually in solid contact with the steel at the bottom of the groove. It is actually visible to some extent on all of the bullets in the photos

The black streaks you are referring to, is that carbon still left in the barrel after cleaning?

Yes.

 

[Outrider27]

Terminal ballistics appear to be identical to what happens in ballistic gel, but played out over as much as twice the distance (depending on velocity) and of course you don't get a record of the terminal path like you do in gel.
[/quote]

Just a random thought, since you have tight control over the impact area; to get a record of the terminal path like you do with gel, mount a digital video camera in a waterproof housing off to one side. Odds are that will give you a good record of the development and dimensions of the terminal path with a little experimentation with camera placement and maybe a yard stick or other object of known dimensions clamped in the field of view for size reference.

 

[David Bookstaber]

Terminal ballistics appear to be identical to what happens in ballistic gel, but played out over as much as twice the distance (depending on velocity) and of course you don't get a record of the terminal path like you do in gel.

Just a random thought, since you have tight control over the impact area; to get a record of the terminal path like you do with gel, mount a digital video camera in a waterproof housing off to one side. Odds are that will give you a good record of the development and dimensions of the terminal path with a little experimentation with camera placement and maybe a yard stick or other object of known dimensions clamped in the field of view for size reference.

Yeah, that does work better than one might expect because even though the bullet moves too fast for consumer-grade cameras the temporary cavities persist long enough to see, and also cast shadows in direct sunlight, as shown here:

For example, looking at the recovered bullets at the beginning of this thread you'd think they'd be terrible for hunting -- like they must just punch a clean hole with no expansion. The water test shows the same thing we see on real animals: they destabilize and tumble immediately, causing a permanent wound channel more characteristic of their length (and of course worse if they hit and break apart on bone).

 

[Ggmac]

JRS check out Medford rifling . Rounded lands and grooves . Cordite is listed as its demise .



Sorry , I see I'm a little late .

 

[Joe Salt]

I've alway wondered about those grooves in the sides of the bullet, while in flight? And don't anyone say it has nothing to do with its flight. Because if the tip alters its flight so do the grooves!

joe Salt

 

[David Bookstaber]

I've always wondered about those grooves in the sides of the bullet, while in flight? And don't anyone say it has nothing to do with its flight. Because if the tip alters its flight so do the grooves!

Another good question. I would think that if nothing else they would accentuate spin drift, since they must give the bullet more "bite" into the air as it's spinning. But I've never seen a ballistic calculator offer a correction value for "spin friction," so I wonder if it's a measurable effect? E.g., comparing the most slippery jacket shot through a polygonal barrel that barely scratches it to a rough jacket shot through a barrel that cuts such deep grooves that the jacket material actually starts to petal outward.

 

[Joe Salt]

I guess I'm going to have to do some digging on our back bank and find out whether any of the bullets I find have any of the copper missing on the sides of the bullets. At the RPM these things are going and a piece comes off that would surely cause a flyer. Kind of like the ones that come apart and never make it to the target.

Joe Salt

 

[Canadian bushman]

Bryan litz discusses the grooves in a bullet and the effect on its flight pretty thoroughly in modern advancements for long range shooting. I really enjoyed the chapter on stability. It was much more involved than i expexted.

He even discussed it a little on the forum in a thread about spin decay.

After getting an idea of the effort and research that went into his studies for the book i wouldnt expect him to give too much away.

 

[David Bookstaber]

Bryan litz discusses the grooves in a bullet and the effect on its flight pretty thoroughly in modern advancements for long range shooting.

Yeah, Bryan says, "The purpose of my testing was to determine how much difference there was in spin rate decay for different types of riflings, and barrels at different levels of wear. Long story short, there is quite a bit of difference." Unfortunately the only practical way to determine it is to measure it.

Which brings me back to the earlier question: It sounds like ballistic calculators should have a variable for spin drift. I guess long-range shooters are just used to including that in their DOPE, and anyone depending on calculators is content with +/- a few inches spin drift at 1000 yards.

 

[Canadian bushman]

I dont think slight deviations in spin rate decay due to rifling would affect the actual amount of spin drift to a significant amount, inside the supersonic region. Not for a bullet with sufficient stability, but im just estimating i dont know for sure.

And like you said if it were a whole inch or two, what kind of shooter would you have to be to notice?

I think the biggest observations that can be made recovering fired bullets is the reduction of integrity of the jacket due to engraving and heat, and the bullets ability to obturate to the bore.

I.E. I have a button rifled barrel that has very sharp edges on the rifling that cuts very deep into jackets, and also creates a lot of heat just off the edge of these cuts. This rifle always produces exceptional expansion, even with thick jacketed bullets moving slow.

 

[Joe Salt]

Mr. bushman its the two inches I worry about at 1000 yards that has always made me wonder about the bullet getting squashed and grooved coming out of the barrel after I have hand picked every last one. And they all look so pretty before that happens. ;D you never know when a chunk may come off, or the grooves pushing it off course!

Joe Salt

 

[Canadian bushman]

If the grooves left by rifling have an effect within a couple of inches of spin drift at 1k, i would think coming out of the same barrel they would be at least consistent with each other, and easily compensated for.

Again i was under the belief that spin drift is caused by tipping off or the bullet steering the direction of its spin not surface friction caused by rifling.

If it were random dispersion causing a few inches in any given direction this is much more of an issue to be worried about, and rifling causing this is more than likely defective or shot out.

Again im not claiming to fully understand this, just sharing how i connect this logic.

 

[David Bookstaber]

Spin drift is caused by the spin of the bullet and its interaction with the air via friction. We know that the rate of spin affects spin drift. Based on Litz et. al. the engraving and other deformation of the bullet by the barrel also affects spin drift. Of course to the degree the barrel's engraving of bullets is consistent this can be measured and factored into ballistics even if it cannot be readily modeled and predicted. OTOH, if the bullets are being damaged to the point of disintegration then of course you have an insurmountable accuracy problem.

 

[Canadian bushman]

http://www.appliedballisticsllc.com/spindrift.html

 

[Bryan Litz Ballistics]

The rifling engravings have nothing to do with spin drift.

As Canadian Bushman (and others) have stated, spin drift is a result of the bullet pointing to the side and steering the trajectory. Imagine if the bullet flew point forward all the way to the target; how could the riflings generate any lateral displacement?

Of course there is a connection; the riflings are an artifact of the bullet needing to spin, and it's the spin that eventually causes spin drift... But if you could fire a bullet with the correct RPM's and not leave riflings engraved on it, it would still experience spin drift.

-Bryan

 

[Joe Salt]

Ok maybe I'm saying this wrong! I've seen bullets come apart in the air, so all I'm wondering, what happens when one is on the verge of coming apart but is still making it to the target. Does it make a difference in flight? So if one rifling is rougher than another does it cause the bullet to start coming apart. What I have seen does not happen all the time but it happens.

Joe Salt

 

[Canadian bushman]

Ok maybe I'm saying this wrong! I've seen bullets come apart in the air, so all I'm wondering, what happens when one is on the verge of coming apart but is still making it to the target. Does it make a difference in flight? So if one rifling is rougher than another does it cause the bullet to start coming apart. What I have seen does not happen all the time but it happens.

Joe Salt

I dont have any first hand experience with this exact scenario but i would think so. Id also go on a limb and assume the accuracy would be bad leading up to, and including this point.

Ive only ever heard of this happening with light bullets leaving barrels going very fast with rather tight twist. Ive also heard shooters credit this situation to barrels. Sometimes crediting the same barrel brand and type that i own, and experience abnormally good terminal effects with. Only with much heavier bullets going much slower.

 

[Joe Salt]

Yes barrels! That's why with a new barrel the first thing I do is JB the crap out of it before I fire a shot. To try and get it as smooth as I can, who knows with everything new you try you may stumble on to another trick. Bryan's article on spin drift I knew about its just the small pieces that may come off the bullet in flight that I've always wondering about. Maybe people using high speed cameras have seen this happen.

Joe Salt

 

[David Bookstaber]

The rifling engravings have nothing to do with spin drift.

As Canadian Bushman (and others) have stated, spin drift is a result of the bullet pointing to the side and steering the trajectory. Imagine if the bullet flew point forward all the way to the target; how could the riflings generate any lateral displacement?

Of course there is a connection; the riflings are an artifact of the bullet needing to spin, and it's the spin that eventually causes spin drift... But if you could fire a bullet with the correct RPM's and not leave riflings engraved on it, it would still experience spin drift.

-Bryan

There is no spin drift* in a vacuum, so it does have to do with the interaction of the spin and the atmosphere. The argument here is that the nature of that interaction could change depending on what the rifling does to the exterior of the bullet.

Consider one extreme example: rifling that cuts vanes into the bullet. Compared to a "regular" bullet at identical muzzle velocity and spin (and ignoring spin decay) the vaned projectile will exhibit astonishing spin drift. In fact over a long flight my understanding is that it will corkscrew with a net drift that is down and to the right.

What we're noting is that sharp engraving probably does something between "nothing" and the "vaned" example, and wondering in practice how far from the "nothing" case it can get.


*Clarification: My understanding is that two separate physical phenomena are often rolled up into the term "spin drift." One is nutation (or gyroscopic drift), which does occur in a vacuum. The other is atmospheric drift, which varies with atmospheric density, and which is the subject of this conversation. (This may include the magnus force and, depending on whether engraving is significant, other aerodynamic "lift" forces like those typically discussed in finned projectiles.)