Randy Robinet, a question about stability.

[jackie schmidt]

I figure you know as much, if not more than most on bullet stability and twist, so here is a question.

Lately, while hiding out at The Tomball Gun Club on Sunday’s, I have been shooting just about every thing I own. A few Sundays ago, I was shooting my HV 6PPC with the usual 66 grn bullets out of the Krieger 13.5 barrels.

On a whim, I decided to stick some 80 grn bullets in it, just to see. I first tried 4895, but could not get it to shoot that well. So Did a little math and figured about 27.5 grns of 133 might work.

It worked and then some.

Yesterday, I had my LV, same 13.5 Krieger, and tried the same load. I had to play with the tuner a tad, but soon had it shooting mid “ones” to small “twos”.

No pressure signs, easy bolt lift. I figure the velocity is around 3150.

From what I have understood, this should not be happening. The Bart’s 80 is a regular short range BT that measures around .945 inch long.

So, how close to the ragged edge am I on bullet stability. We looked close at the bullet holes, and they look perfect.

I kind of look at this combo as something to reach for when the regular 6PPC just refuses to cooperate. That is, unless bullets start going through the paper sideways.

 

[R.G. Robinett]

Jackie, as you suspect, the combination of a BT bullet, of this length and a 13.5" twist barrel, is, "a wreck waiting to happen": Sg 0.9 - technically, Sg less than 1.0 is unstable. At sea-level, Std conditions, 3300fps, a 6mm BT of that length, for Sg 1.5, needs a 1:10.4" twist. I'd certainly never expect this combo to perform, nor, would I deliberately try to duplicate it. This one defies the math, then, moons it going past!

The long-time 6mm standard model, .845" long, FB 6mm bullet & 1:14" twist, produces something <Sg 1.3 . . . the addition of a BT dictates almost a full inch faster twist rate . . . so, a typical 68gr BT, via a 1:14" twist BBL results in < Sg 1.2! WE know, that most of the time, this sorta, kinda "works" . . . . if, one buys enough BBLs to obtain one which defies the math. RG

 

[jackie schmidt]

Randy, years ago, in the late 1990’s, a well known Gulf Coast Region Shooter won the Crawfish with a new barrel on the market, Turbo. He was shooting a 15 twist.

I decided to get one. At that time we were shooting the great Fowler 66 grn FB on .825 jackets. Our usual barrels were 1-14.

I chambered that 15 twist barrel, and the bullets were going through the target almost sideways.

Something must be wrong, so we checked the actual twist with a cleaning rod and a tight patch. It turned out to be closer to 1-15 1/2 twist. A friend had some 65 grn Watson’s made on .790 jackets. They shot great.

Around that time,many were talking about 1-14 really being too slow for the 65 grn bullet on .825 jackets. Many times I would notice the first shot on a target being a lot bigger than it should be.

That is when the 13.5 twist started getting real popular. I switched to them in the early 2000’s and have not used anything else since.

I am going to take my Rail Gun to the range this week end. It has a new 13.5 Krieger that shot really well a month or so ago with a variety of 66 and 68 gr bullets. I am going to put these 80’s through it and see what happens.

You mentioned altitude. We are not far above sea level in our area, does twist become more critical or less the higher above sea level you get?

 

[geo.ulrich]

Jackie , can you test some in New Orleans then you would be below sea level.....

 

[R.G. Robinett]

Jackie , can you test some in New Orleans then you would be below sea level.....

Yer killin' me George!RG

 

[Al Nyhus]

I certainly never expect this combo to perform, nor, would I deliberately try to duplicate it. This on defies the math, then, moons it going past!

Randy, you and I have talked about these strange anomalies quite a few times in the past...round holes when the math doesn't add up. I've witnessed it on several occasion.

The hypothesis that 'in-bore-yaw' becomes the factor that tips edgy combinations to either side of the 'do not cross' threshold is the only reasonable explanation. At least to my Quantum Physics trained prefrontal cortex

Since you're as familiar as anyone with Robert McCoy and Bill Davis' work, the McDrag/McGyro/McTwist studies and JMB's stuff....do you feel there is a default IBY value used in the computations? Or were they figured with what would essentially be zero IBY?

Good shootin'! -Al

 

[R.G. Robinett]

Randy, years ago, in the late 1990’s, a well known Gulf Coast Region Shooter won the Crawfish with a new barrel on the market, Turbo. He was shooting a 15 twist.

I decided to get one. At that time we were shooting the great Fowler 66 grn FB on .825 jackets. Our usual barrels were 1-14.

I chambered that 15 twist barrel, and the bullets were going through the target almost sideways.

Something must be wrong, so we checked the actual twist with a cleaning rod and a tight patch. It turned out to be closer to 1-15 1/2 twist. A friend had some 65 grn Watson’s made on .790 jackets. They shot great.

Around that time,many were talking about 1-14 really being too slow for the 65 grn bullet on .825 jackets. Many times I would notice the first shot on a target being a lot bigger than it should be.

That is when the 13.5 twist started getting real popular. I switched to them in the early 2000’s and have not used anything else since.

I am going to take my Rail Gun to the range this week end. It has a new 13.5 Krieger that shot really well a month or so ago with a variety of 66 and 68 gr bullets. I am going to put these 80’s through it and see what happens.

You mentioned altitude. We are not far above sea level in our area, does twist become more critical or less the higher above sea level you get?

Presuming sea-level and Std conditions you're about as bad as it gets - well, except for temperature - THAT can be a make, or, break attribute. You may recall the "winter league" matches/ PS articles from way back in the late 80's - somewhere in the far NE, my memory just ran dry regarding exactly where - describing bullet/barrel combinations (particularly BT bullets) which shot great in summer temps, then, going to hell in sub 10* F temps.

The ragged-edge is a dangerous place to reside: cold air is DENSE. What is the actual twist of the current 1:13.5"? If you're interested, later this evening, I'll run some calculations where just the temp. varies: you can just about disregard humidity, as it has very little affect on air density. RG

P.S. Memory just kicked in - Dunham's Bay winter league!!

 

[R.G. Robinett]

Randy, you and I have talked about these strange anomalies quite a few times in the past...round holes when the math doesn't add up. I've witnessed it on several occasion.

The hypothesis that 'in-bore-yaw' becomes the factor that tips edgy combinations to either side of the 'do not cross' threshold is the only reasonable explanation. At least to my Quantum Physics trained prefrontal cortex

Since you're as familiar as anyone with Robert McCoy and Bill Davis' work, the McDrag/McGyro/McTwist studies and JMB's stuff....do you feel there is a default IBY value used in the computations? Or were they figured with what would essentially be zero IBY?

Good shootin'! -Al

Al, waaaAy back, beyond my ability to recall his identity, my 6 degree of freedom guru, from MIT, would never use less than 0.0001" of in-bore-yaw . . . his predictions usually ran neck-and-neck with/verified Tioga and JBM. But, as we're aware, strange stuff happens! RG

 

[jackie schmidt]

Presuming sea-level and Std conditions you're about as bad as it gets - well, except for temperature - THAT can be a make, or, break attribute. You may recall the "winter league" matches/ PS articles from way back in the late 80's - somewhere in the far NE, my memory just ran dry regarding exactly where - describing bullet/barrel combinations (particularly BT bullets) which shot great in summer temps, then, going to hell in sub 10* F temps.

The ragged-edge is a dangerous place to reside: clod air is DENSE. What is the actual twist of the current 1:13.5"? If you're interested, later this evening, I'll run some calculations where just the temp. varies: you can just about disregard humidity, as it has very little affect on air density. RG

Randy, these are cut rifled Kriegers, I assume they are dead on.

The one on my HV is an old 13.5 that is probably circa 2007. I chopped the chamber off, (it was always a good barrel), and rechambered it at just over 20 inches.

Here are two groups I shot Sunday with the LV. The one on the right is about .160, I resighted the rifle and shot the one on the left. Conditions were a straight tail wind probably around 6 to 8 mph. The single shot on the S is a single shot to confirm sight in. That hole is pretty darned round.

The LV is a new blank I chambered for last years Nationals but did not shoot it. I opted for another barrel on the LV.
This is two Kriegers, 13.5 twist, that are close to 13 years apart in age. Both shot the 80 quite well.

http://benchrest.com/attachment.php?attachmentid=23517&stc=1&d=1587588335

If the Rail Gun barrel shoots the 80 grn this week end, I will just have to chalk it up to the alignment of Saturn’s largest moon Titan and it’s relationship to Pluto

 

[BillBrawand]

way back when

Randy, these are cut rifled Kriegers, I assume they are dead on.

The one on my HV is an old 13.5 that is probably circa 2007. I chopped the chamber off, (it was always a good barrel), and rechambered it at just over 20 inches.

Here are two groups I shot Sunday with the LV. The one on the right is about .160, I resighted the rifle and shot the one on the left. Conditions were a straight tail wind probably around 6/8 mph.

The LV is a new blank I chambered for last years Nationals but did not shoot it. I opted for another barrel on the LV.
This is two Kriegers, 13.5 twist, that are close to 13 years apart in age. Both shot the 80 quite well.

http://benchrest.com/attachment.php?attachmentid=23517&stc=1&d=1587588335

If the Rail Gun barrel shoots the 80 grn this week end, I will just have to chalk it up to the alignment of Saturns largest moon Titan and it’s relationship to Pluto

in 73 bullets shot were 70 gr remingion came out with 68 grthat came on like gang busters I made bunch of cores jackets came heavy 71.5 blts shot like hell 1 14 twist

 

[Steve Kostanich]

Concerning in barrel bullet yaw

At least 25 years or more back, there was National Geographic special shown on PBS, that was on high speed photography, showing how slowing down fast moving objects or speeding up slow events could be photographed.

Naturally, projectiles were featured subject matter.

The one that is significant here was the photographing of a 50 BMG bullet coming down the barrel. They had a system of mirrors and lights set up so you could see the bullet coming down the barrel, which actually shot the mirror. The bullet had an "X" striped around the ogive and point to use as a rotational reference.

In the fraction of a second you got to see the bullet, it was pretty clear the bullet was yawing slightly.

I don't know if or where that program is available, but you all might find it interesting. They don't comment on the yawing at all as I recall.

FWIW
Steve Kostanich

 

[R.G. Robinett]

in 73 bullets shot were 70 gr remingion came out with 68 grthat came on like gang busters I made bunch of cores jackets came heavy 71.5 blts shot like hell 1 14 twist

Bill, roger that - however, those bullets were made using <.800" long jackets, thus finished OAL of about 0.81".
In about 1985/6, Mike Walker wrote an open letter to PRECISION SHOOTING Editor, Dave Brennan, which Dave published for all the BR world to read.

Ed Spivey, and the SpiveCo, Inc., gurus, original makers of J4 jackets, had decided to make the "standard" [68gr] jacket 0.825" in length. Quite elegantly, without criticism, or, lengthy mathematics, Mr. Walker pointed out that, for 1:14" twist barrels, and 68gr 6mm FB bullets, the jacket should not exceed 0.800" long . . . and, as they say, the rest is history . . . For some 35 years, BR shooters have lived on the ragged edge, and only recently, taken 1/2 step in the right direction . . .

That extra 0.025" in OAL is worth about 1.00" of twist rate requirement for what became the "classic" 68gr FB, which is why, ever since, the 14" twist has been a hit-or-miss proposition - that is, buy enough barrels and eventually get one which defies the math.

Or, as did a few savvy individuals, such as FRED HASECUSTER, shoot bullets made using the .750", and/or, .790" long jacket . . . I recall several times, at the OLD St. Louis range, people asking Mr. Hasecuster, "what are you shooting" - he'd answer, followed by the classic reply, "I tried that, and it doesn't work . . . Fred Hasecuster is in the Hall of Fame, and they that aren't!

All that said, comprehending what Mike Walker told us, required some reading and study - Mr. Walker knew of what he spoke. Sg is more about length than weight, the latter of which, is mostly , "along for the ride" . . . RG

 

[R.G. Robinett]

At least 25 years or more back, there was National Geographic special shown on PBS, that was on high speed photography, showing how slowing down fast moving objects or speeding up slow events could be photographed.

Naturally, projectiles were featured subject matter.

The one that is significant here was the photographing of a 50 BMG bullet coming down the barrel. They had a system of mirrors and lights set up so you could see the bullet coming down the barrel, which actually shot the mirror. The bullet had an "X" striped around the ogive and point to use as a rotational reference.

In the fraction of a second you got to see the bullet, it was pretty clear the bullet was yawing slightly.

I don't know if or where that program is available, but you all might find it interesting. They don't comment on the yawing at all as I recall.

FWIW
Steve Kostanich

Steve, good reference. In the book, Modern Exterior Ballistics, by Robert McCoy, there are, shadow graph reproductions clearly showing post muzzle exit yaw/pitch - not quite as high tech, but, none the less, quite clearly happening!

One attribute which BR shooters have found difficult to embrace, is the very nearly perfect jackets we have today: relative to what the founders, in the Seattle, WA and Johnstown, NY, had to use in the late 1940s, today, there is NO need to fear increasing group size via twist rates which impart reliable Sg1.4 or, better. And, by going here, one will get a far higher percentage, of [so called] hummers. RG

 

[glp]

Randy

Steve, good reference. In the book, Modern Exterior Ballistics, by Robert McCoy, there are, shadow graph reproductions clearly showing post muzzle exit yaw/pitch - not quite as high tech, but, none the less, quite clearly happening!

One attribute which BR shooters have found difficult to embrace, is the very nearly perfect jackets we have today: relative to what the founders, in the Seattle, WA and Johnstown, NY, had to use in the late 1940s, today, there is NO need to fear increasing group size via twist rates which impart reliable Sg1.4 or, better. And, by going here, one will get a far higher percentage, of [so called] hummers. RG

I'm not ta home right now, but there is a recent article in, I think, Reloader magazine(or Rifle) that talks about shock wave as the bullet exits the muzzle, flat base vs. boattail. The author has photos showing the shock wave behind the FB but ahead of the BT. Not specific to this topic but closely related re accuracy perhaps.

 

[Tim Singleton]

May just be imagining it but
In my 14 and 13.5 twist barrels the .790 seems to make a smaller hole than the .825
The 790 measures about .228 or so the .825 hole is .237 or so

Jackie
What does a single bullet hole measure with the 80gr

 

[jackie schmidt]

May just be imagining it but
In my 14 and 13.5 twist barrels the .790 seems to make a smaller hole than the .825
The 790 measures about .228 or so the .825 hole is .237 or so

Jackie
What does a single bullet hole measure with the 80gr

Tom, go back to the pictures in my post #9. There is a single bullet on that “S”. I measured ir with dial calibers, it does measure right at .243.

 

[R.G. Robinett]

"Randy, these are cut rifled Kriegers, I assume they are dead on.

The one on my HV is an old 13.5 that is probably circa 2007. I chopped the chamber off, (it was always a good barrel), and rechambered it at just over 20 inches."

Jackie, though the cut barrels are very uniform, I have seen two Kriegers which were miss-marked - the [actual] twist rates were not close . . . that said, both, for bullets other than originally intended, were quite good. So, I'm extremely curious and desirous to have you measure the actual twist rate of that barrel - if not faster than 13.5, it's a mathematical miracle! RG

 

[BillBrawand]

read corbins article on rebate bullrts

I'm not ta home right now, but there is a recent article in, I think, Reloader magazine(or Rifle) that talks about shock wave as the bullet exits the muzzle, flat base vs. boattail. The author has photos showing the shock wave behind the FB but ahead of the BT. Not specific to this topic but closely related re accuracy perhaps.

it says rebate bulleted most accurate

 

[BillBrawand]

try this now

Bill, roger that - however, those bullets were made using <.800" long jackets, thus finished OAL of about 0.81".
In about 1985/6, Mike Walker wrote an open letter to PRECISION SHOOTING Editor, Dave Brennan, which Dave published for all the BR world to read.

Ed Spivey, and the SpiveCo, Inc., gurus, original makers of J4 jackets, had decided to make the "standard" [68gr] jacket 0.825" in length. Quite elegantly, without criticism, or, lengthy mathematics, Mr. Walker pointed out that, for 1:14" twist barrels, and 68gr 6mm FB bullets, the jacket should not exceed 0.800" long . . . and, as they say, the rest is history . . . For some 35 years, BR shooters have lived on the ragged edge, and only recently, taken 1/2 step in the right direction . . .

That extra 0.025" in OAL is worth about 1.00" of twist rate requirement for what became the "classic" 68gr FB, which is why, ever since, the 14" twist has been a hit-or-miss proposition - that is, buy enough barrels and eventually get one which defies the math.

Or, as did a few savvy individuals, such as FRED HASECUSTER, shoot bullets made using the .750", and/or, .790" long jacket . . . I recall several times, at the OLD St. Louis range, people asking Mr. Hasecuster, "what are you shooting" - he'd answer, followed by the classic reply, "I tried that, and it doesn't work . . . Fred Hasecuster is in the Hall of Fame, and they that aren't!

All that said, comprehending what Mike Walker told us, required some reading and study - Mr. Walker knew of what he spoke. Sg is more about length than weight, the latter of which, is mostly , "along for the ride" . . . RG[/QUOTE in 1980 I had made up bag gun for unlv shot 63 gr on 740 jacket deadly later on 810 825 same shot ever since 0ver size 2437

 

[alinwa]

"Randy, these are cut rifled Kriegers, I assume they are dead on.

The one on my HV is an old 13.5 that is probably circa 2007. I chopped the chamber off, (it was always a good barrel), and rechambered it at just over 20 inches."

Jackie, though the cut barrels are very uniform, I have seen two Kriegers which were miss-marked - the [actual] twist rates were not close . . . that said, both, for bullets other than originally intended, were quite good. So, I'm extremely curious and desirous to have you measure the actual twist rate of that barrel - if not faster than 13.5, it's a mathematical miracle! RG

Good call

and GOOD thread!