Smooth Bore Spin Stabilization? Rifled slugs?

[faustus]

Ok, I just felt like sharing this ....

So, it seems there is a bit of a controversy out there.

Are rifled slugs like the Brenneke or a rifled Foster slug more accurate because of spin stabilization? And to be honest ... after doing a lot of reading on the subject .... and after today's result I don't know any more ....

I have a cantilever Hastings Wadlock barrel ... and which has straight grooves ... and in order to prevent a projectile from spinning.

I tried slugs out of this barrel in the past ... but I never got good results ... and I thought it is because the slug can't rotate in the barrel .... and I gave up on that idea ... and in my mind ... that barrel was a shot or buckshot only barrel.

So, today ... just for the heck ... I tried some Federal TruBall slugs out of that barrel .... and to my surprise the grouping was really not bad.

Here the barrel with a red dot mounted:



And here the result at 50 yards:



Specs:
- 12ga Hastings Wadlock barrel
- Improved Cylinder Choke
- distance 50 yards/meters
- 2 to 2 1/2 inch group
- Federal 12ga TruBall slugs
- 3 shot group

With that result ... I am not sure any more what to think of rifled slugs spin stabilization ...

I mean that barrel prevents a projectile from spinning while passing through the barrel ... So, if there is any spin stabilization happening ... it would happen after the slug left the barrel ... and that just seems .... hmmmmm ... lets say very unlikely ...



Thoughts anyone?

[longbow]

I would have agreed not long ago and even Brenneke states that the helical ribs on their slugs does not cause any rotation. However... Taofladermaus has shot video of rifled slugs rotating in the air. The rotation is slow so certainly not fast enough for spin stabilization but enough to correct for extreme deviations in flight so instead of veering off one way the slug takes a spiral flight much like helical fletching on an arrow which keeps slightly bent shafts on a relatively true course.

https://www.youtube.com/watch?v=jRPo19DWlZY

But no, rifling on slugs does not cause rotation in the barrel. It is air drag in flight that causes rotation. I will disagree with Taofledermaus there! What would cause the slug to spin in the bore? If there was heavy fouling in the bore that might do it as the ribs cut fouling but with a slug pressing against the clean bore there is no rotational force, no drag or friction component to cause rotation.

Several factory rifled slugs have shot very well for me, so much so it is annoying that my home cast and loaded slugs generally do not perform as well. I believe a good portion of the problem is in the wad column. Factories can spec what they want and buy by the 10's of thousands so can be very consistent with loads and wad columns... and they are using new hulls.

Also, Federal Tru-Ball has a pretty good reputation for accuracy so you picked a good one to test.

I am not clear on exactly what wadlock barrels do because I find it hard to believe that a wad full of shot rotates in the barrel... what would cause rotation? Bore friction is high so whatever causes wad rotation has to work at it. Not saying that wadlock barrels don't improve patterns, they obviously do or they wouldn't exist but just how does that wad rotate without wadlock? Or maybe it is the tighter bore of wadlock barrels tightening patterns?

Longbow

[RMc]

After some searching, I located this 1991 DOJ/FBI Report which documents slow rotation as a contributing factor to rifled slug accuracy:


"The slight rotation imparted by the ribs reduces the effect of manufacturing irregularities. In tests performed by Winchester-Western, the slug rotation was confirmed, resulting in consistently smaller groups for rifled slugs than unrifled slugs…"

"The basic Foster slug, however, remains a
hollow lead cup, heavier at the point. It has 14
small angled ribs swaged into the side of the
slug. The rifling tends to be obliterated by the
passage of the slug through the barrel, especially
through a full choke. Some spin does result
from the ribs, however, and tests show a very
slow spin of approximately one turn in 24 feet
of travel to one turn in 129 feet of travel,
depending upon the choke used...

Neither Brenneke nor Foster slugs depend
upon the rifling ribs or projectile spin for stability.
The slugs are stable because they travel
through the air like a sand-filled sock with the
heavier toe forward (O'Connor 1965), unlike
symmetrical lead balls (Figure 3). The trailing
light end acts as a stabilizer. The slight rotation
imparted by the ribs reduces the effect of manufacturing
irregularities. In tests performed
by Winchester-Western, the slug rotation was
confirmed, resulting in consistently smaller
groups for rifled slugs than unrifled slugs
(Sterett 1966)."

Crime Laboratory, Digest April 1991 - Volume 18, No.2

Full report here:

https://www.ncjrs.gov/pdffiles1/Digi...32229NCJRS.pdf

What I find most interesting is the varying rate of spin noted for different amounts of choke constriction.

[Boomer81]

After some searching, I located this 1991 DOJ/FBI Report which documents slow rotation as a contributing factor to rifled slug accuracy:


"The slight rotation imparted by the ribs reduces the effect of manufacturing irregularities. In tests performed by Winchester-Western, the slug rotation was confirmed, resulting in consistently smaller groups for rifled slugs than unrifled slugs…"

"The basic Foster slug, however, remains a
hollow lead cup, heavier at the point. It has 14
small angled ribs swaged into the side of the
slug. The rifling tends to be obliterated by the
passage of the slug through the barrel, especially
through a full choke. Some spin does result
from the ribs, however, and tests show a very
slow spin of approximately one turn in 24 feet
of travel to one turn in 129 feet of travel,
depending upon the choke used...

Neither Brenneke nor Foster slugs depend
upon the rifling ribs or projectile spin for stability.
The slugs are stable because they travel
through the air like a sand-filled sock with the
heavier toe forward (O'Connor 1965), unlike
symmetrical lead balls (Figure 3). The trailing
light end acts as a stabilizer. The slight rotation
imparted by the ribs reduces the effect of manufacturing
irregularities. In tests performed
by Winchester-Western, the slug rotation was
confirmed, resulting in consistently smaller
groups for rifled slugs than unrifled slugs
(Sterett 1966)."

Crime Laboratory, Digest April 1991 - Volume 18, No.2

Full report here:

https://www.ncjrs.gov/pdffiles1/Digi...32229NCJRS.pdf

What I find most interesting is the varying rate of spin noted for different amounts of choke constriction.

The varying spin would, in my mind, be because as you get tighter and tighter choke, those helical lines grab harder and thus impose more spin on the slug (all be it very slightly in comparrison to a sabot out of a rifled barrel). Open choke = barely grabbing the rear fins. Full choke = tight grip thus imposing slightly more spin.

[Hogtamer]

Rmc and Longbow seem to have this right as usual. I would suspect that many of those soft lead "rifled"slugs down the bore would produce major leading of the barrel. Sure nice groups though, and yes, the trueball loads do have a good reputation for accuracy.

[GBertolet]

Is wadlock really intended for slugs? Just asking. I was under the impression it was to keep shotcups from rotating, allegedly preventing shot from dispersing more. Something like turkey hunters would use with fine shot for headshots on gobblers. Seems to work ok for slugs though.

[9.3X62AL]

Wadlock was marketed as a shotcup rotation preventer, meant for shot load dispersion control. Does the idea work? Dunno.

I have shot a BUNCH of 12 gauge Foster slugs over the years. My usual grouping with rifle-sighted Rem 870 barrels at 50 yards is in the 3"-4"/5-shot ballpark.

[Ballistics in Scotland]

I don't think Longbow's or the FBI/DOJ's judgements would surprise anybody with a serious interest in the subject. A spirally grooved slug can indeed spin, but to an extent that can't possibly stabilise it.

Archers did indeed discover the benefits of spiral feather fletchings, to spin the arrow. But the reason for this being beneficial is quite different. An irregularly shaped arrow will deviate to one side. That will be a curved path, like a boat with the helm over, rather than straight-line, so it may worsen drastically at long range. If it spins, it will deviate to every direction in turn, and describe a corkscrew path, which might be the same at all ranges to which the arrow travelled. As long as the diameter of the spiral was smaller than, just for supposing's sake, a Frenchman, it could be perfectly satisfactory.

Almost certainly this inspired the first use of rifling, for the wrong reasons, just as cauterising a wound to give any entering devils a hard time could bring rather uncomfortable sterilisation. But that isn't gyroscopic stabilisation like a rifle bullet must have. If we take the length and diameter of an arrow, and any of the calculations of required twist developed in the firearms age, we could probably twist the arrow in two before achieving that.

The same could be said of the slow-spinning shotgun slug: deviation converted into a corkscrew path, rather than gyrostabilisation. The difference, though, is that it is difficult to bend a slug. A smooth and regular nose and skirt are probably just as good.

[Ballistics in Scotland]

Wadlock was marketed as a shotcup rotation preventer, meant for shot load dispersion control. Does the idea work? Dunno.

I have shot a BUNCH of 12 gauge Foster slugs over the years. My usual grouping with rifle-sighted Rem 870 barrels at 50 yards is in the 3"-4"/5-shot ballpark.

This isn't unusual, with various types of slug. The trouble is that groups at 100, with any kind of smoothbore projectile, tend to be a lot more than twice as big.

I also think the slight spin a rifled slug can achieve arises in the air, and not from friction in the bore or choke.

[9.3X62AL]

This isn't unusual, with various types of slug. The trouble is that groups at 100, with any kind of smoothbore projectile, tend to be a lot more than twice as big.

I also think the slight spin a rifled slug can achieve arises in the air, and not from friction in the bore or choke.

I have seen similar outcomes past 50 yards. At 100 yards, most shots stay in the black on a B-27 target--with more than half inside the 8-ring. Our regimen largely said "Pick up the rifle" once engagement ranges exceeded 50 yards.

[bikerbeans]

Shotshells for Germans and arrows for the French. I don't care for your gun control laws but I like your hunting seasons.

BB

[Ballistics in Scotland]

Shotshells for Germans and arrows for the French. I don't care for your gun control laws but I like your hunting seasons.

BB

Well, my wife is German, and she has more non-antique shotguns than I do. I tend to think of Colonel Fremantle on the field of Gettysburg - although I've got his book, and he had neither red coat, teacup nor official position. Talking of civil war and the consequent beheadings, he says "of course we wouldn't dream of doing it nowadays."

[Blood Trail]

I would have agreed not long ago and even Brenneke states that the helical ribs on their slugs does not cause any rotation. However... Taofladermaus has shot video of rifled slugs rotating in the air. The rotation is slow so certainly not fast enough for spin stabilization but enough to correct for extreme deviations in flight so instead of veering off one way the slug takes a spiral flight much like helical fletching on an arrow which keeps slightly bent shafts on a relatively true course.

https://www.youtube.com/watch?v=jRPo19DWlZY

But no, rifling on slugs does not cause rotation in the barrel. It is air drag in flight that causes rotation. I will disagree with Taofledermaus there! What would cause the slug to spin in the bore? If there was heavy fouling in the bore that might do it as the ribs cut fouling but with a slug pressing against the clean bore there is no rotational force, no drag or friction component to cause rotation.

Several factory rifled slugs have shot very well for me, so much so it is annoying that my home cast and loaded slugs generally do not perform as well. I believe a good portion of the problem is in the wad column. Factories can spec what they want and buy by the 10's of thousands so can be very consistent with loads and wad columns... and they are using new hulls.

Also, Federal Tru-Ball has a pretty good reputation for accuracy so you picked a good one to test.

I am not clear on exactly what wadlock barrels do because I find it hard to believe that a wad full of shot rotates in the barrel... what would cause rotation? Bore friction is high so whatever causes wad rotation has to work at it. Not saying that wadlock barrels don't improve patterns, they obviously do or they wouldn't exist but just how does that wad rotate without wadlock? Or maybe it is the tighter bore of wadlock barrels tightening patterns?

Longbow

Jeff, the guy behind the camera and the brains of Taofledermous is a good friend of mine and a regular contributor on my Buck and Slug Reloading page on Facebook.

We went round and round on this issue. The rifling is all but swaged out once the slug leaves the barrel, so any rotation is incidental and don’t affect anything.

I went and dug up a report from the FBI ballistics lab that says rifling in foster slugs don’t cause any significant spin to affect accuracy. I’m going with that.
https://www.ncjrs.gov/pdffiles1/Digi...32229NCJRS.pdf



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[RMc]

After some searching, I located this 1991 DOJ/FBI Report which documents slow rotation as a contributing factor to rifled slug accuracy:


"The slight rotation imparted by the ribs reduces the effect of manufacturing irregularities. In tests performed by Winchester-Western, the slug rotation was confirmed, resulting in consistently smaller groups for rifled slugs than unrifled slugs…"

"The basic Foster slug, however, remains a
hollow lead cup, heavier at the point. It has 14
small angled ribs swaged into the side of the
slug. The rifling tends to be obliterated by the
passage of the slug through the barrel, especially
through a full choke. Some spin does result
from the ribs, however, and tests show a very
slow spin of approximately one turn in 24 feet
of travel to one turn in 129 feet of travel,
depending upon the choke used...

Neither Brenneke nor Foster slugs depend
upon the rifling ribs or projectile spin for stability.
The slugs are stable because they travel
through the air like a sand-filled sock with the
heavier toe forward (O'Connor 1965), unlike
symmetrical lead balls (Figure 3). The trailing
light end acts as a stabilizer. The slight rotation
imparted by the ribs reduces the effect of manufacturing
irregularities. In tests performed
by Winchester-Western, the slug rotation was
confirmed, resulting in consistently smaller
groups for rifled slugs than unrifled slugs
(Sterett 1966)."

Crime Laboratory, Digest April 1991 - Volume 18, No.2

Full report here:

https://www.ncjrs.gov/pdffiles1/Digi...32229NCJRS.pdf

What I find most interesting is the varying rate of spin noted for different amounts of choke constriction.

Jeff, the guy behind the camera and the brains of Taofledermous is a good friend of mine and a regular contributor on my Buck and Slug Reloading page on Facebook.

We went round and round on this issue. The rifling is all but swaged out once the slug leaves the barrel, so any rotation is incidental and don’t affect anything.

I went and dug up a report from the FBI ballistics lab that says rifling in foster slugs don’t cause any significant spin to affect accuracy. I’m going with that.
https://www.ncjrs.gov/pdffiles1/Digi...32229NCJRS.pdf

Sent from my iPhone using Tapatalk

A most interesting divergence of conclusions based on the same report!

[jeremy360]

Has anyone here made their own wedge or version of a tru-ball just to test accuracy (throwing economics out the window here of course)?

[longbow]

I think so but don't recall details. Corbin sells a variety of plastic balls and I think there was another source someone was at least suggesting for use. I know there was discussion but I don't recall if there were tests.

http://www.corbins.com/bball.htm

Yeah, looks like Corbin tops out 3/8" which would be too small.

These may work:

http://www.plasticballs.com/plastic-...propylene.html

or these:

http://www.hooverprecisionplastics.c...tic-Balls.html

I haven't taken any Tru-Ball rounds apart to measure slug diameter but have read that Federal increased Foster slug diameter to nominal bore size. If so the Tru-Ball wouldn't need to expand a lot... not like swelling a Lyman Foster up by 0.025" anyway.

If you have a Foster slug mould it is not a bad idea to try if you can find a plastic ball to fit the cavity. Not sure if there is any advantage over a slightly oversize slug that swages down but the Tru-Balls do seem to have a reputation for good accuracy.

Go for the gusto and give it a try Jeremy! Don't forget to post pics.

Longbow

[faustus]

A Federal TruBall taken apart .... it has a very solid wad and the ball that centers the slug ....

[Hogtamer]

Those photos are very revealing. Nothing special about the slug but !wow!what a wad. Solid column hard plastic for a straight launch and they obviously believed that centering the wad for a perfect launch is the key to accuracy. The ball being lower than the slug also promotes quick and uniform obturarion. We lowly home loaders can only drool at that.

[Blood Trail]

Those photos are very revealing. Nothing special about the slug but !wow!what a wad. Solid column hard plastic for a straight launch and they obviously believed that centering the wad for a perfect launch is the key to accuracy. The ball being lower than the slug also promotes quick and uniform obturarion. We lowly home loaders can only drool at that.

We’ll find a way.


What’s the diameter of that ball?


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[faustus]

Ok, I just measured the ball with calipers ....

It is 0.4980 inches in diameter ...

And the wad is harder than any of my other wads ... it is even harder than the SPW/ Hammerhead Slug wad from slugsrus.com ...

Harder meaning ... I can't compress it with my fingers or when leaning on it with my body weight ... I can't deform it a bit with just finger pressure .... whereas all my other wads will deform to some extend ...