Effects of Rifling Grooves on Performance

[THEIS]

Hi,

I wonder with the new projectile technologies we have today that if this study was performed with modern monolithic projectiles what the conclusion would be:

https://apps.dtic.mil/dtic/tr/fulltext/u2/a505719.pdf

Sincerely,
Theis

 

[lash]

Hi,

I wonder with the new projectile technologies we have today that if this study was performed with modern monolithic projectiles what the conclusion would be:

https://apps.dtic.mil/dtic/tr/fulltext/u2/a505719.pdf

Sincerely,
Theis

Have you been following Warner Flatline studies?
Who am I kidding? Of course you have. Your question was rhetorical.

 

[THEIS]

Have you been following Warner Flatline studies?
Who am I kidding? Of course you have. Your question was rhetorical.

Hi,

LOL I referring to the actual grooves, not the twist rates

Over a decade ago Lost River Ballistics done all the tests anyone cared to read in regards to twist rates with their solids...long story short....
7 twist everything lolol

Sincerely,
Theis

 

[Geno C.]

There was that super slow motion video of a bullet in flight posted not that long ago. In the shadow you could see the ripple affect the rifling had on the surrounding air. So some affect so seen id imagine. A sabot round might be interesting to see the difference.

 

[steve123]

A friend who has shot hundreds of thousands of pellets through many different types of airguns barrels found that polygonal rifling made pellets have less wind drift.

He won the Field Target nationals in 2017 with a Walther match barrel using that type of rifling. He laughs at me when I tell him how far out I have to aim in the wind to hit the same kill zone, lol, which is a third more.

Just commenting...

 

[THEIS]

Hi,

@steve123
That is intriguing. Do you know if the groove depths and widths are the same in regards to his poly rifled pellet rifle barrel compared to your standard rifled pellet rifle barrel?

Sincerely,
Theis

 

[lash]

A friend who has shot hundreds of thousands of pellets through many different types of airguns barrels found that polygonal rifling made pellets have less wind drift.

He won the Field Target nationals in 2017 with a Walther match barrel using that type of rifling. He laughs at me when I tell him how far out I have to aim in the wind to hit the same kill zone, lol, which is a third more.

Just commenting...

Okay, so I’ve thought about this, and not being a physicist but having read up a bit on stuff in general, I have a couple of questions.

First is this. Since studies seem to inducate that spin drift is directly correlated to the rpms of a projectile with relation to its velociity, why would there be less spin drift using polygonal rifling? Secondly, would that not seem to indicate that the polygonal rifling is not actually spinning the projectile as fast as a similar twist land and groove rifling would?

 

[steve123]

THEIS and Lash,

Since there is just the small surface area of the head and skirt of the pellet that touches the rifling. He or anybody for that matter, is able to push the pellet through the barrel with a cleaning rod. He uses some blunt thingamajig on the end of the rod so the pellet is undamaged and falls onto a rag.

He has many barrels he tests, some with no choke, some with slight choke, and some with quiet a bit of choke, and with varied twists and profiles. Of course off the top of my head I can't remember all the results but I do recall seeing the pellets pushed out of his winning barrel. There was very little engraving on the surfaces that touch the rifling and where it touches those areas are a more rounded slight engraving.

Basically from one extreme to another some of the pellets from other various barrels with regular rifling have aggressive and deep rifling cuts/engraving, some pretty normal looking, and some not very deep on the head and skirt.

All what I mentioned above didn't mean some of those barrels weren't accurate, some of them were very accurate and he won state with them. They just caused pellets to blow in the wind normally is all.

What drew his attention originally to this phenomenon was he shot a 12ftlb energy level PCP air rifle, going back and forth, against his 20ftlb energy level PCP air rifle, and noticed the weaker air rifle had a little less wind drift than the more powerful one. Scratching his head and wondering how that was even possible he started experimenting.

Past what I wrote here I'm in the dark as far as theory or understanding about this subject. My friend Bobby is a salt of the earth type guy, very intelligent, etc, and the last thing he'd do is make things up, so there you have it.

 

[lash]

Steve, don’t misunderstand my questions. I am not doubting in any way the veracity of your friend’s statements nor yours. I am just very interested in the whys and wherefores.

Ugg say, knowledge good, ignorance bad.

 

[steve123]

Steve, don’t misunderstand my questions. I am not doubting in any way the veracity of your friend’s statements nor yours. I am just very interested in the whys and wherefores.

Ugg say, knowledge good, ignorance bad.

I didn't take your post negatively and I think most reading my post are questioning the "why's" or "if's" ??!!

Actually if I didn't know Bobby personally I'd have my doubts.

In the sport Field Target air rifle, past 30Y, it's mostly about reading wind and pulling the trigger at the moment the wind is right. Like in a 5-6 mph full value wind at 55Y with a 20ftlb rifle shooting at a 1.5" kill zone, which is actually really a 1.5" KZ minus the width of the pellet (targets won't fall if the pellet touches the edge of the faceplate resulting in a miss), I would need to aim .3-.4" out of the KZ for a center hit. Bobby could get away with aiming just inside the edge to get a hit, and he has more of a fudge factor to still get a hit if the wind changes.

A 10.3 gr JSB .177 pellet has a .030 BC so it will blow about 5 times as much as a .135 BC 40gr 22 rf bullet going 150 fps faster! FT matches can be very challenging in high winds!

 

[Lynn Jr]

THEIS
What about gain twist barrels and there engraving?

 

[lash]

Hi,

LOL I referring to the actual grooves, not the twist rates

Over a decade ago Lost River Ballistics done all the tests anyone cared to read in regards to twist rates with their solids...long story short....
7 twist everything lolol

Sincerely,
Theis

Okay, I’ll admit that before, I merely skimmed the article and then commented. Since you called me on it, I spent a bit more time to actually read and digest it a bit. So, while we all know (or assume) that the rifling engraving on solids differs from conventional clad projectiles and there seems to be some evidence that clad bullets can internally deform somewhat due to that engraving, the question seems valid. However, reading the conclusions that were reached from this 2008 study, I have to wonder if the results would vary significantly at all.

“The computational show the relative effect between smooth and grooved (rifled) projectiles to be minimal over a range of spin rates and that the rifling grooves do not produce aerodynamic effects responsible for the observed trim angles.. The technique was then applied to a current generation small-caliber projectile. In conjunction with a spark range test, the effect of the rifling grooves and spin rate (constant twist versus inflight pD/V) at both launch and downrange velocities on the aerodynamic coefficients and derivatives were found to either be minimal or within the experimental error of the ballistics testing.”

There was that super slow motion video of a bullet in flight posted not that long ago. In the shadow you could see the ripple affect the rifling had on the surrounding air. So some affect so seen id imagine. A sabot round might be interesting to see the difference.

Geno, the study did in fact compare both pristine and pre-engraved bullets in sabots as comparisons. Results were as quoted above.

THEIS
What about gain twist barrels and there engraving?

This also seems like a good question, but unless the resulting engraving differs significantly from a standard twist rate barrel, any differences would likely still fall within the margins of error of the testing capability.

Still, I’m all for more testing if done in a consistent and controlled manner such as the test we are discussing. It’s only our tax dollars at work...lol.

 

[Lynn Jr]

How is a gain twist the same as a conventional twist?
Let's start out at 10 twist and go to 7 twist?
Wouldn't the engraving change with the twist rate?
Or are you saying it is so small as to not matter?

 

[lash]

How is a gain twist the same as a conventional twist?
Let's start out at 10 twist and go to 7 twist?
Wouldn't the engraving change with the twist rate?
Or are you saying it is so small as to not matter?

I did not say that they are the same, at all. What I said was, that unless the engraving differs significantly from standard twist rifling engraving, that, based upon the results of the study posted in the OP, the differences might well be lost in the noise of the testing error.

I agree that it makes sense that the engraving should differ, I just don’t know to what extent. If I recall correctly, someone did post some pictures of rifling from a standard twist versus a gain twist, I’m just not sure where those are right now.

Also, don’t think that I am stating an absolute, since I have no facts to back up such a claim. I am merely posing the thought that it may not make a measurable difference, based solely on the published results of the study linked in the OP.

 

[Lynn Jr]

Lash
I don't know either so hopefully Mr Boatright will chime in.
My thinking is that when you go with a gain twist you are constantly changing the engraving on the bullet until it leaves the barrel and that the engraving starts out small and grows the entire time it's in the bore.

 

[THEIS]

Hi,

Here is image of projectile recovered from a gain twist barrel..courtesy of google search via Lutz Moeller website from 2006 lol.

Notice how wide the drive bands are eaten at the front of projectile compared to the rear.
A traditional twist has the same width of eaten bands from front to back.

Sincerely,
Theis

 

[Lynn Jr]

Theis
That is exactly what my concern would be. How does that much loss of support affect accuracy given we are shooting 60,000 psi loads?

 

[lash]

Lash
I don't know either so hopefully Mr Boatright will chime in.
My thinking is that when you go with a gain twist you are constantly changing the engraving on the bullet until it leaves the barrel and that the engraving starts out small and grows the entire time it's in the bore.

You’re going to make me do the math aren’t you?

I’m no physicist, but the basics of what you are describing is just really the angular differences between the starting twist rate and the ending twist rate at muzzle.

I’m on my way out the door right now, so give me a bit to get somewhere that I can work that out and I’ll post back.

Edit: I see the pic from Theis, and it seems evident as he describes. So I guess we’re back to the original question as posted then.

 

[lash]

Theis
That is exactly what my concern would be. How does that much loss of support affect accuracy given we are shooting 60,000 psi loads?

I think the realistic answer is, what does the bullet tell you? Are the actual results on target consistent and repeatable?

 

[Jim Boatright]

My understanding is that for either solid lead-alloy black-powder-style bullets or for jacketed lead-cored rifle bullets, the gain twist rifling engraving marks on recovered bullet shanks look about the same as they would if fired from constant-twist barrels at the final twist-rate. In other words, the bullets shanks themselves twist about their spin-axes as the helix angle of the rifling increases down the gain-twist barrel, so the engraving marks remain almost unchanged. For lead alloys, this twisting is almost entirely plastic, that is, "permanent." The thin gilding metal jacket is not that much stouter than its lead-alloy core.

On the other hand, monolithic copper bullets absolutely do not need gain-twist barrels at any super-fast twist-rates I have tried (20-calibers per turn). I have an 18-caliber twist (6.0-inch twist, 5R rifling) Bartlein 338 barrel on order. Rifling engraving is very clean with almost no detectable differences between the driving-side and back-side faces. Gain-twist only applies with lead-alloy or, possibly, with lead-cored bullets.

I concluded long ago, as the study in the OP confirms, that the engraved rifling marks produce no significant aerodynamic effects chiefly because they are submerged in the turbulent boundary layer which isolates them from the major aerodynamics of the rifle bullet. The engraving of the shorter 0.7-caliber top width of the rear driving bands of my copper bullets would certainly produce even smaller (more negligible?) aerodynamic effects.

I am aware of the limit cycle yawing of most conventional jacketed, lead-cored rifle bullets in transonic flight. This was thought by McCoy to be exacerbated by radiusing the rear corners of the bullet bases (which helps avoid bullet destabilization in the muzzle-blast zone). I radiused the entire base of my latest Mark IIb design (convex) to minimize muzzle blast disturbances and kept the back corners sharp to minimize limit cycle yawing in the transonic. Only further test-firings can tell if the design is successful. It is already the most aerodynamically efficient supersonic rifle bullet design by a large margin. My 335-grain 375-caliber Mk IIb copper bullet should match the measured BC of 0.479 announced by Berger for their new 379-grain copper ELR bullet, and it can be fired much faster from any given rifle.

 

[Jim Boatright]

Hi,

Here is image of projectile recovered from a gain twist barrel..courtesy of google search via Lutz Moeller website from 2006 lol.
View attachment 7008949
Notice how wide the drive bands are eaten at the front of projectile compared to the rear.
A traditional twist has the same width of eaten bands from front to back.

Sincerely,
Theis

This looks to be a monolithic copper-alloy bullet. The shear strength of the bullet material was a little too much for the bullet to twist quite as much as the helix angle increased in the gain-twist barrel. As I have said repeatedly, he need not have bothered with a gain-twist barrel for his copper bullets.

 

[oneshot.onehit]

Here’s a little entertainment

 

[oneshot.onehit]

Gain twist 16 - 7.7 414 grain GS recovered a few years back at a 2000 yard target from one that missed low and stayed in good shape.

 

[Jim Boatright]

RE: Oneshot's recovered 375 copper bullet fired from the gain-twist Bartlein barrel.

It looks like the widths of the land engraving marks shown in the gas sealing rings increase by about 50 percent from bottom to top along the shank of the copper bullet. The helix angle started at 4.21 degrees and ended at 8.61 degrees, for a factor of 2.04 faster twist-rate at the muzzle. This copper bullet had far greater radial contact pressure against the insides of the barrel at the base of its shank than it had farther forward. Radial gas-sealing contact pressure is mostly caused in this bullet design by elastic expansion of the copper bullet caused by its inertial acceleration produced by the base-pressure behind that bullet. The internal (axial-direction) compressive stress at any point along the accelerating bullet's length depends upon how much bullet mass is ahead of that point in the bullet, decreasing to zero at the very tip. The copper material of the boat-tail, which is behind the obturating disc at the rear of the shank, is actually experiencing tensile axial stress as it is being dragged forward by its attachment to the shank.

This is why the bullet stayed locked into the rifling pattern at the base of the shank (where barrel obturation actually occurs). The shank of the bullet both partially twisted and partially widened the engraving marks farther forward on the shank as the barrel's twist-rate tightened. The twisting of the copper bullet was elastic, so it un-twisted right after leaving the barrel, leaving the rifling engravement marks at their initial slow-twist 4.2-degree helix angle along the left sides of the engravement marks. If one forced a soft lead slug through this same barrel from the chamber end, it would show the rifling marks only as wide as the rifling lands and spiraling at the final 8.6-degree angle because that lead slug would have been permanently twisted (plastically) by the gain-twist rifling.

The left sides of the grooves (as pictured) line up at the initial 4.2-degree helix angle in the G-T barrel and are heavily smeared into the relief groovings. That would actually be the non-driving side of the rifling lands. This smearing happened very early at peak base-pressure when the greatest spin-up torque was also being applied. The initial smearing on the right-hand (driving) sides of the engraving marks seems to have later been worn away, or "eaten" as Theis said.

That's what I am seeing here, anyway.

Jim Boatright

 

[_Windrider_]

Hi,

LOL I referring to the actual grooves, not the twist rates

Over a decade ago Lost River Ballistics done all the tests anyone cared to read in regards to twist rates with their solids...long story short....
7 twist everything lolol

Sincerely,
Theis

I’ve been saying that for years.

 

[Lynn Jr]

On the other hand, monolithic copper bullets absolutely do not need gain-twist barrels at any super-fast twist-rates

Jim
This is what I needed to know. I can't see smearing the bullet like in the first photo from Theis as being a good thing.

 

[Jim Boatright]

Gain-twist rifling was developed in the 1800's for shooting heavy cast or swaged lead-alloy slugs (subsonic bullets) at black powder velocities for which it works fairly well. I would restrict jacketed, lead-cored bullets to slower twist-rates where gain-twist is not needed. I cannot see gain-twist rifling ever being needed for firing properly designed monolithic copper rifle bullets.

EDIT: I have rewritten, corrected, and expanded my hastily worded reply in Post #24 above considering Oneshot's recovered copper bullet, so please re-read it if you wish to.

 

[TiroFijo]

Most interesting topic, gentlemen... thanks all.