Rotational decay and its effect on LR stability.

[wadcutter]

Can anyone enlighten me on the pros and cons of rifling induced, rotational decay to manage bullet stability?

I have heard two conflicting descriptions. One saying to use shallow rifling to reduce rotational decay as a higher maintained spin rate helps stability.

The other thought is that the deeper rifling helps reduce spin at a rate closer to the forward decay in velocity. This helps the bullet better follow the path of flight and avoids a nose up attitude that leads to a sudden catastrophic failure of flight near the transonic barrier.

 

[groper]

Re: Rotational decay and its effect on LR stability.

Everything you want to know on this subject is covered in here;

http://www.patentstorm.us/patents/6629669/description.html

 

[TiroFijo]

Re: Rotational decay and its effect on LR stability.

Or at least <span style="font-style: italic">claimed</span> there...

I suspect there are more important issues to deal with at transonic speed than than rotational speed, and that (as usual) it varies a lot depending on the specific bullet design.

 

[The Mechanic]

Re: Rotational decay and its effect on LR stability.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wadcutter</div><div class="ubbcode-body">Can anyone enlighten me on the pros and cons of rifling induced, rotational decay to manage bullet stability?

I have heard two conflicting descriptions. One saying to use shallow rifling to reduce rotational decay as a higher maintained spin rate helps stability.

The other thought is that the deeper rifling helps reduce spin at a rate closer to the forward decay in velocity. This helps the bullet better follow the path of flight and avoids a nose up attitude that leads to a sudden catastrophic failure of flight near the transonic barrier. </div></div>
The later is true. It is the Cheytac design and seems to be proven out but the specific bullets, rifling, MV, and other factors have to all be in zen.

 

[groper]

Re: Rotational decay and its effect on LR stability.

I think i know what you are eluding to wadcutter, i remember your shooting results whereby the 300SMK wouldnt remain stable thru transonic, whereas the the 250grain tipped bullet did.

Im assuming your considering a new barrel, and would like to maximize your chances of keeping that 300gr bullet stable thru transonic, so your wondering what sort of rifling profile to get in this new barrel that will help most in this regard?

The answer is not easy as we need to know what that bullet is doing when it goes transonic - only available thru dopplar radar data. It could be that its pitching moment rises to significantly high levels to destabilze it - in which case we would want to minimize its RPM decay with minimal rifling so the gyroscopic inertia can overcome it. Or, it could be that its flying with an increased angle of repose and the aerodynamic forces and physicial inertia properties of that bullet, increase the yaw precession to the point of dynamic instability in which case, no small differences in RPM decay will help. Some bullets, cannot remain stable thru transonic no matter how much spin they have, its implicit in their aerodynamic characteristics in the transonic regime... i cannot say if the 300SMK is one of these bullets, however the 300gr Scenar has proven stable thru transonic from a 1:10twist to 2700yds - simple way to find out, is ask other people whove shot it way out there and see if they maintained good accuracy - or just switch from the SMK to the 300scenar and try again from your current barrel.

Easiest method to determine whether is over (or under) spin stabilized at the end of its flight, and therefore loosing stability this way, is to simply ask around and find out who is having more success beyond transonic distances, the guys with 1:10 barrels or the guys with 1:9-9.5 barrels. The difference should be evident, perhaps a new thread "whos shot the 300SMK to +2000yds? and then asking what barrel twists and rifling profiles they used and accuracy achieved, would yield a pattern that may shed light on your query?

 

[marduk185]

Re: Rotational decay and its effect on LR stability.

have to say just in awe at some of the physics yall get into on here. awesome.

 

[BryanLitz]

Re: Rotational decay and its effect on LR stability.

This exact subject is currently being discussed here:
http://www.usrifleteams.com/lrforum/inde...amp;#entry98915

To address the .338 300 and 250 grain options specifically...

I gather the 250 grain (SMK and Scenar) are rock solid, stability wise thru transonic in all conditions. However, the 300's can be 'iffy'. Length, specifically the ratio of transverse to axial inertia, is the biggest difference between these bullets as it relates to stability.
The biggest affect that determines how well the 300 grain bullets make it thru will be atmospherics. Without running the numbers, I would make an educated guess that a 1000 foot difference in elevation would improve the transonic stability of this bullet more than going from a 1:10" to a 1:9.35" twist. Unfortunately you don't always get to choose the altitude of the target.

-Bryan

 

[bohem]

Re: Rotational decay and its effect on LR stability.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: groper</div><div class="ubbcode-body">...is to simply ask around and find out who is having more success beyond transonic distances, the guys with 1:10 barrels or the guys with 1:9-9.5 barrels. The difference should be evident, perhaps a new thread "whos shot the 300SMK to +2000yds? and then asking what barrel twists and rifling profiles they used and accuracy achieved, would yield a pattern that may shed light on your query? </div></div>

I've seen it work and not work at 2200yd from the same rifle with the same load on the same target.

Rifle is a 9.5tw Kreiger tube, 30" long on a BAT Machine receiver. In the winter (29F) at 1650 ASL (28.6 inHg) the bullets would not repeatably hit the 34" square at 1930yd but they would punish the 24" square at 1800yd. And by 2200yd you were lucky to see a dirt splash within 20yd of the target.

The same setup was shot at 4200' ASL (26.2 inHg) last summer I watched the very same rifle, same owner, load, everything, clobber the 34" plate with high repeatability at 2198yd

All the ranges were calculated via GPS numbers, so they're +/- 6yd with the receiver I own. That was close enough for what we were playing around with.

The conclusion that I made from this test (remember, same ammo, same rifle, same shooter, big change in DA) was that the 300 SMK is borderline trannsonic stable. Depending upon the conditions it may or may not work.

This is similar to how the 30c 168 SMK behaves from a 308 in a 10tw. In certain areas it will run down to about Mach 1.2-1.25 and then fall apart, but under different conditions I've seen it stay stable well past the sound barrier as evidenced from hitting with it at 1250yd.

ETA: Good Morning, Bryan!

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Length, specifically the ratio of transverse to axial inertia</div></div>

At the time I was watching the tests I hadn't read Bob McCoy, when Francis and I were working through the chapters on stability the equations suddenly validated many conclusions we'd come to from just watching...

Those pesky Ixx Iyy and how they fit into the stability equations are very important.

 

[BryanLitz]

Re: Rotational decay and its effect on LR stability.

Good morning bohem!

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Those pesky Ixx Iyy and how they fit into the stability equations are very important.</div></div>

Very much so.

In fact, the relationship between Ixx and Iyy as it relates to stability is the root cause of an often misunderstood fact about bullet stability.

Most believe that moving the CG forward in a spin stabilized design will improve stability because you're closing the distance between the CG and CP, thereby reducing Cma. However, by adding weight to the nose, what you're also doing is increasing Iyy much more than Ixx. The result is that usually, adding weight to the forward part of the bullet actually makes the bullet less stable, not more stable because the ratio of Iyy/Ixx increases and has a greater destabilizing effect than the fact that you've reduced Cma by a little bit.

-Bryan

 

[VJJPunisher]

Re: Rotational decay and its effect on LR stability.

I dont understand any of this shit.......boolit shoot good or boolit shoot bad

 

[TiroFijo]

Re: Rotational decay and its effect on LR stability.

Bryan, and what's your take on the "controlled spin" theory?

It was granted a US patent, but it is one of those things difficult to defend in practice... you would have to test the bullets in doppler radar to check rotational speed and compare to your claims, or perhaps someone could come up with examples of previous designs that behaved pretty much the same thus rendering the patent not valid.

By the way, LRB is long gone, and now cheytac too, does anybody knows who owns the patent nowadays?

 

[McCrazy]

Re: Rotational decay and its effect on LR stability.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: VAJayJayPunisher</div><div class="ubbcode-body">I dont understand any of this shit.......boolit shoot good or boolit shoot bad </div></div>

I am with VJJ on this one. It is great that there are people that understand this science and can make improvements to the equipment we shoot with. However I am also grateful I can just load some projectiles and have them work without having to understand why.

 

[BryanLitz]

Re: Rotational decay and its effect on LR stability.

TiroFijo,

From the thread linked above, here are my thoughts on the CheyTac controlled spin:

"Regarding Chey-Tac's claim of 'controlled spin', I don't buy it, and for many reasons. First, their claim implies that they have, by design, created a projectile with specific spin decay properties. I know the limitations of how accurately state of the art aero prediction tools can predict the roll damping coefficient (and other aero coefficients related to said design), and the accuracy simply isn't there to predict spin rate decay to the level that would be necessary. Also, a bullets nose won't point higher late in the trajectory if it's more stable, it points more to the right (for a right twist barrel). This is called yaw of repose, and is the well understood mechanism behind spin drift. Finally, even if a projectile with 'controlled spin' were perfectly designed as they describe it, it would only work 'right' in one atmospheric condition. Go off that condition, and all the factors that result in the perfect balanced spin stability scenario are different; way different when you traverse 1000's of feet of elevation.

A bullet will remain stable at transonic/subsonic speed if certain gyroscopic (static) and dynamic stability criteria are met. The faster a bullet is spinning as it enters transonic speed, the better it's chances of remaining stable will be, all else being equal. So the idea of 'putting the brakes' on spin by just the right amount to aid downrange stability is non-sense unless what they're doing is minimizing the roll damping coefficient to produce maximum retained spin. This is not what's implied by their projectile design, <span style="font-style: italic">and is not even a property of the bullet, it's a property of the barrel and what rifling it leaves engraved on the bullet.</span>"

-Bryan

 

[Emouse]

Re: Rotational decay and its effect on LR stability.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TiroFijo</div><div class="ubbcode-body">Bryan, and what's your take on the "controlled spin" theory?

It was granted a US patent, but it is one of those things difficult to defend in practice... you would have to test the bullets in doppler radar to check rotational speed and compare to your claims, or perhaps someone could come up with examples of previous designs that behaved pretty much the same thus rendering the patent not valid.

By the way, LRB is long gone, and now cheytac too, does anybody knows who owns the patent nowadays?</div></div>


US Patent,..easy way to get complete nonsense passed that just will not hold water. Its sheer smoke and mirrors.

 

[Lindy]

Re: Rotational decay and its effect on LR stability.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">...you would have to test the bullets in doppler radar...</div></div>

I know a little something about radar. I'm skeptical that any radar can measure the spin rate of a small-arms projectile.

 

[groper]

Re: Rotational decay and its effect on LR stability.

The dopplar radar doesnt measure the spin, amongst other things, it can see the bullet yaw... from the epicyclic yaw charateristics observed, you can make very good estimations of MANY of the aerodynamic properties of that bullet, including its roll damping moment and thus how much spin it has left in it, at any point down range...

 

[wadcutter]

Re: Rotational decay and its effect on LR stability.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: groper</div><div class="ubbcode-body">I think i know what you are eluding to wadcutter, i remember your shooting results whereby the 300SMK wouldnt remain stable thru transonic, whereas the the 250grain tipped bullet did.

Im assuming your considering a new barrel, and would like to maximize your chances of keeping that 300gr bullet stable thru transonic, so your wondering what sort of rifling profile to get in this new barrel that will help most in this regard?

The answer is not easy as we need to know what that bullet is doing when it goes transonic - only available thru dopplar radar data. It could be that its pitching moment rises to significantly high levels to destabilze it - in which case we would want to minimize its RPM decay with minimal rifling so the gyroscopic inertia can overcome it. Or, it could be that its flying with an increased angle of repose and the aerodynamic forces and physicial inertia properties of that bullet, increase the yaw precession to the point of dynamic instability in which case, no small differences in RPM decay will help. Some bullets, cannot remain stable thru transonic no matter how much spin they have, its implicit in their aerodynamic characteristics in the transonic regime... i cannot say if the 300SMK is one of these bullets, however the 300gr Scenar has proven stable thru transonic from a 1:10twist to 2700yds - simple way to find out, is ask other people whove shot it way out there and see if they maintained good accuracy - or just switch from the SMK to the 300scenar and try again from your current barrel.

Easiest method to determine whether is over (or under) spin stabilized at the end of its flight, and therefore loosing stability this way, is to simply ask around and find out who is having more success beyond transonic distances, the guys with 1:10 barrels or the guys with 1:9-9.5 barrels. The difference should be evident, perhaps a new thread "whos shot the 300SMK to +2000yds? and then asking what barrel twists and rifling profiles they used and accuracy achieved, would yield a pattern that may shed light on your query?

</div></div>

Groper,
You are right on the money. My 1:10 twist barrel has issues with the 300 gn SMK at ER. I have just recieved a 1:9.3 Benchmark barrel and was wondering how it would effect the 300 gn SMK. Also going to order a .375" barrel soon and after reading cheytac White paper I was thrown into a spin so to speak.

Also have some issues with shooting the 155gn HBC bullet out of my .308 F-Class rifle and was considering the best barrel twist options for 1200 yards matches.

Bryan,
Thanks for your information. What you say adds a new perspective.

 

[Greg Langelius *]

Re: Rotational decay and its effect on LR stability.

I think that this pretty much boils down to the same basic admonition that when one chooses to employ ballistic implements in regimes at the edges of stability, bad things can happen. Simply put, if you want problems, stick your neck out. Conservative approaches allow one to concentrate on the basics, rather than on the extremes.

On another subject, my belief is that the attrition among specialized fabricators is at least as much a product of a strained economy as of the validity of any particular ballistic strategy they had devised. Right now, folks are backing off purchasing dream items, and concerns who concentrate solely on providing such products are competing for a share of a dwindling market. They either diversify,or fold.

Greg

 

[bohem]

Re: Rotational decay and its effect on LR stability.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wadcutter</div><div class="ubbcode-body">

...
My 1:10 twist barrel has issues with the 300 gn SMK at ER. I have just recieved a 1:9.3 Benchmark barrel and was wondering how it would effect the 300 gn SMK.
... </div></div>

Do you have notes on your atmospheric conditions with the 10tw @ extended range when it demonstrated those issues? I ask for the reason that I've posted a band of conditions where issues existed, maybe with your data we can cut that band down to a narrower region to better quantify the behavior of the 300 SMK.

 

[Driftwood]

Re: Rotational decay and its effect on LR stability.

Mongo's head hurt....boolit shoot good or boolit shoot bad?

 

[wadcutter]

Re: Rotational decay and its effect on LR stability.

The atmospherics were about 14 C and 994mb.
I did a write up on it and it can be read here... Extreme range

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Wadcutter</div><div class="ubbcode-body">

...
My 1:10 twist barrel has issues with the 300 gn SMK at ER. I have just recieved a 1:9.3 Benchmark barrel and was wondering how it would effect the 300 gn SMK.
... </div></div>

Do you have notes on your atmospheric conditions with the 10tw @ extended range when it demonstrated those issues? I ask for the reason that I've posted a band of conditions where issues existed, maybe with your data we can cut that band down to a narrower region to better quantify the behavior of the 300 SMK. </div></div>

 

[bohem]

Re: Rotational decay and its effect on LR stability.

Cool, thanks.

For those not versed in C and millibar, this is:

14C ~ 57F
994mB ~ 29.74 inHg

This is pretty close to STP (standard temperature and pressure) conditions of 59/29.92 inHg and conditions are actually worse than the lower altitude pressure/temp conditions that I mentioned above.