Slow load for transonic stability evaluation?

[phlegethon]

One of the difficulties of ELR shooting is that it’s hard to evaluate how well bullets handle the transition from supersonic to subsonic flight. Litz and others have commented that this may often be worse with higher BC bullet designs. Has anyone tried running very light loads such that the transition to subsonic may occur earlier, say 500-600 yards, to reduce the effect of wind and other environmentals? Aside from finding the load, what would go wrong with this approach? I’m sure it’s been tried before. @Jim Boatright @THEIS any comments?

 

[gnochi]

You want the bullet spinning at the same RPM, which doesn’t decrease too much over the course of flight, which means an insanely fast twist barrel - on the order of 1:3-1:4 inches.

Furthermore, to make sure you have a safe load, you need to use a much smaller cartridge.

I think a 375 Raptor at 1:4 twist investigating a bullet for 375 Cheytac with a 1:8 twist would probably be about right, but you’re limited by a bullet weight that’s slightly supersonic with the Raptor with good ballistics for the Cheytac.

 

[phlegethon]

Yes, that makes sense.

 

[Steel head]

So far I can’t say I’ve seen anything unusual with lead core Bullets.
Cheap ones always seem to get squirrelly but that’s probably a high BC variation.
I’m about to delve into the world of solids and Litz did I believe say it’s the longer Bullet that have more issues transitioning

 

[addertooth]

You should swim in some of the air rifle forums. They work with the transonic to subsonic issues all the time with slugs. I tend to tune my High Power Air rifles to be slightly below the transonic range to avoid the destabilization of the round as it passes through the supersonic barrier... things get a bit unpredictable with objects in that region.

 

[THEIS]

Hi,

@addertooth
He isn't talking about "tuning" the ammunition to find the MV to avoid destabilization. Nobody in the ELR realm is going to "tune" their ammunition to purposely be so slow as to avoid transonic destabilization.

What he is asking is if anyone has used the "slow the MV down" approach to test the stabilization/destabilization of particular projectiles at more readily available distances and to reduce the environmental conditions impacts on said stabilization/destabilization.

There have been some ballistics companies say this is how they can account for testing stabilization/destabilization at such short distances and with doppler and acoustic equipment that is only designed for such short distances.

But as @gnochi pointed out...the twist rates would have to be severely modified on those "testing" systems to account for the RPMs needed to realistically "mimic" the true stabilization/destabilization zones as the real MV ammunition.


Sincerely,
Theis

 

[addertooth]

Hi,

@addertooth
He isn't talking about "tuning" the ammunition to find the MV to avoid destabilization. Nobody in the ELR realm is going to "tune" their ammunition to purposely be so slow as to avoid transonic destabilization.

What he is asking is if anyone has used the "slow the MV down" approach to test the stabilization/destabilization of particular projectiles at more readily available distances and to reduce the environmental conditions impacts on said stabilization/destabilization.

There have been some ballistics companies say this is how they can account for testing stabilization/destabilization at such short distances and with doppler and acoustic equipment that is only designed for such short distances.

But as @gnochi pointed out...the twist rates would have to be severely modified on those "testing" systems to account for the RPMs needed to realistically "mimic" the true stabilization/destabilization zones as the real MV ammunition.


Sincerely,
Theis

I like what you posted, but my point is slightly different. The air rifle guys have also done a lot of work on the super/trans/sub sonic transition of slugs, and what behaviors are seen. They also have looked at the effect of velocity on BC as well. Some of those airgun guys are serious shooters, with the big matches going to the guys who consistently shoot sub 1/4 MOA at 100 yards (with air rifles).

 

[Yerman]

Interesting topic. I’m not sure I understand the twist rate needing to be faster.

If we are talking about a 375ct running a 1:7, by the time the bullet gets to transonic, doesn’t the twist rate slow down in addition to the velocity meaning you would want a slower twist rate at the start to closely mimic what is happening down range?

 

[phlegethon]

Interesting topic. I’m not sure I understand the twist rate needing to be faster.

If we are talking about a 375ct running a 1:7, by the time the bullet gets to transonic, doesn’t the twist rate slow down in addition to the velocity meaning you would want a slower twist rate at the start to closely mimic what is happening down range?

No, rotation is minimally affected by drag, so the bullet will generally spin at essentially the same rate throughout its flight path, even if linear velocity has gone down dramatically.

This is something I knew on some level before posting the question, but didn’t know well enough to make the connection. This is why it’s nice to have true experts to consult.

 

[Steel head]

Interesting topic. I’m not sure I understand the twist rate needing to be faster.

If we are talking about a 375ct running a 1:7, by the time the bullet gets to transonic, doesn’t the twist rate slow down in addition to the velocity meaning you would want a slower twist rate at the start to closely mimic what is happening down range?

My understanding is the RPM’s slow very little compared to fps slowing over distance.
With that I wouldn’t think a big change if any would be needed of the twist rate as basically your new muzzle speed would be the same as say 2200 yards with your bullet going transonic at 2800 as an example.

Then you could go transonic at 600 yards with a far better chance of knowing the conditions it flew through.

 

[vh20]

My understanding is the RPM’s slow very little compared to fps slowing over distance.
With that I wouldn’t think a big change if any would be needed of the twist rate as basically your new muzzle speed would be the same as say 2200 yards with your bullet going transonic at 2800 as an example.

Then you could go transonic at 600 yards with a far better chance of knowing the conditions it flew through.

Dang! What are you shooting that goes transonic at 2800?

 

[Steel head]

A lot of ELR cartridges transition in that area.
The 338 I’m putting together should transition from 2600-2800 depending on the speed I get.

 

[vh20]

There aren’t a “lot” that transition at that range where I live (near sea level). Some, sure. My 338 LM and NM begin transition just under a mile unless it’s blistering hot (>90F) running 300 SMK, 300 Berger, and 285 ELD around 2750-2800 fps. This is assuming we’re talking about the beginning of the transition around 1300-ish fps, where they start to wander.

 

[Steel head]

Where I live (near sea level) my 338 LM and NM begin transition just under a mile unless it’s blistering hot (>90F) running 300 SMK, 300 Berger, and 285 ELD around 2750-2800 fps. This is assuming we’re talking about the beginning of the transition around 1300-ish fps, where they start to wander.

I should have 285 grain solids at 2850-2950 FPS and I generally have a DA of about 3000 at the beginning of ELR season.

My current 7mm transitions at about 2000 at that DA
190’s at 2880.

 

[vh20]

What kind of G7s?

 

[Steel head]

What kind of G7?

Don’t know but G1 is 1.01

 

[vh20]

Hmmm.
By “transonic”, do you mean the specific point at which it goes to subsonic, or (as I do) the beginning of the transonic range (fps in low 1300s)?

 

[gnochi]

A Seneca 212 30cal launched at 2950 (light load for 30 Sherman Magnum / 300 PRC Improved) goes below 1300fps at 2180yd (20degC, 30.07inHg, 23%RH), for sake of comparison.

 

[timintx]

Might add that low loads can be dangerous , it can double the load on the bolt lugs . just under max is where it should always remain to be safe.

Tim in Tx.

 

[phlegethon]

Might add that low loads can be dangerous , it can double the load on the bolt lugs . just under max is where it should always remain to be safe.

Tim in Tx.

I would have tried something like Trail Boss for this, but I think the twist rate renders it moot.

 

[Jim Boatright]

I will try to shed some light on this transonic stability topic.
Ballisticians define gyroscopic stability (Sg) as (P^2)/(4M), where P is basically proportional to the bullet's instantaneous spin-rate (p) and M is sort of the instantaneous overturning moment (trying to tumble the bullet). We have learned to launch our rifle bullets with an initial Sg of 1.5 or greater for best results. Sg increases throughout the bullet's flight in flat firing, especially early on in that flight. The spin-rate slows only gradually (exponentially) with time of flight. The overturning moment is due to an aerodynamic lift force acting laterally at its Center of Pressure (CP) which is almost always ahead of the bullet's Center of Gravity (CG). That lift force is proportional to the square of the airspeed (V) of the bullet through its direct dependence on dynamic pressure (q = 0.5*rho*V^2). The airspeed (V) decreases quickest early in the bullet's supersonic flight, much faster than the spin-rate (p) is decreasing. So, Sg is sort of proportional to the square of p/V during flight with V decreasing faster than p.
The dynamic stability of a rifle bullet refers to whether, or not, its gyroscopic precession (or coning) angle is increasing with ongoing time of flight. A conventional rifle bullet usually cones around at a 2 to 5-degree angle to the airstream through which it is flying. A hyper-stable bullet cones at a minimum coning angle of about 0.1-degree solely due to the gravitation curvature of its flight path. An unstable bullet eventually cones at 90-degrees or more. If you listen to incoming 30-caliber bullets in the pits at a 1,000-yard match, you can hear the difference between stable and unstable rounds. Bullets coning at large angles produce a distinctive whirring sound modulated at their slow (perhaps 30 hz) coning rates.
Supersonic and subsonic bullet flight are quite stable compared to transonic flight. To test transonic stability at short ranges requires firing rifle bullets at those initial velocities, but with initial Sg values of 4.0 to 6.0, which requires very quick twist-rate rifling, to emulate conditions normally occurring far downrange.
I hope this helps. If not, email questions to me at <[email protected]>.
Jim Boatright

 

[chutinlead]

So I get that reducing a round that typically leaves the muzzle at 3000fps to transonic is going to reduce RPM enough to not give you a true idea on what subsonic flight is really like. Now reducing a round that usually goes 1200 to 1000 to see what the BC would be in the subsonic range using something like a Labradar and the JBM calculator I could see giving more realistic data.

 

[gnochi]

So I get that reducing a round that typically leaves the muzzle at 3000fps to transonic is going to reduce RPM enough to not give you a true idea on what subsonic flight is really like. Now reducing a round that usually goes 1200 to 1000 to see what the BC would be in the subsonic range using something like a Labradar and the JBM calculator I could see giving more realistic data.

Not really. RPM has a measurable impact on BC.

 

[Jim Boatright]

I am disturbed whenever I hear bullet airspeeds (V) expressed in feet per second (fps) used in discussing the breakpoints between supersonic, transonic, and subsonic flight regimes. What kind of bullet is it and what are the local atmospheric conditions? Airspeeds must be expressed as Mach speeds to be meaningful in that context. Mach speed is V/a where a is the "speed of sound" in the ambient atmosphere through which the bullet is flying. At sea level (height = 0) in an ICAO Standard Atmosphere, a(0) is 1116.45 fps. The speed of sound (a) varies primarily with ambient air temperature (T) expressed in degrees Fahrenheit: a(0) = (49.0223 fps)*SQRT[T + 459.67] = 1116.45 fps at 59 degrees F. There are also minor secondary corrections for atmospheric pressure and humidity. The sea level ICAO atmosphere is dry (Rh = 0) and dense: standard barometric pressure is 760 mm of Hg (29.92"). The obsolete ASM atmosphere is no longer used in ballistics. The Army Standard Metro atmosphere was developed in the 1880's to minimize hand calculations of ballistic corrections at Aberdeen Proving Grounds in Maryland. Bullet makers persist in using it because it allows slightly (at least 0.5%) larger BC values to be claimed.
As a rifle bullet slows to Mach 1.0 (V = a), its supersonic-dominant bow shockwave has just disappeared. Shockwaves from the afterbody then dominate while the bullet slows to its particular maximum subsonic Mach speed (usually Mach 0.8 to Mach 0.9, depending on the bullet design), where the last (base) shockwave finally disappears. The upper limit of the transonic regime for each bullet is more difficult to define (i.e., just when the first subsonic flow appears in the whole flow-field around that bullet). I generally use the reciprocal of that bullet design's maximum subsonic Mach speed to define the the top end of the transonic range (usually Mach 1.10 to Mach 1.25). The transonic airspeed region is only about half as wide for "super streamlined" Ultra-Low-Drag (ULD) rifle bullets as compared with conventional match bullets.
Jim Boatright

 

[steve podleski]

If you use your existing barrel for slower muzzle velocities, you can determine the effect on Sg: lower velocity mean lower spin rate but also lower turning moment. You can determine the effect on Sg by taking ratios between standard muzzle velocities Sg and reduced muzzle velocity Sg (spin is linear with velocity but moment is proportional to the square of velocity as stated by Jim Boatright. This may give you a worst case effect.

 

[Jim Boatright]

If you use your existing barrel for slower muzzle velocities, you can determine the effect on Sg: lower velocity mean lower spin rate but also lower turning moment. You can determine the effect on Sg by taking ratios between standard muzzle velocities Sg and reduced muzzle velocity Sg (spin is linear with velocity but moment is proportional to the square of velocity as stated by Jim Boatright. This may give you a worst case effect.

Yes, initial Sg is theoretically independent of muzzle velocity V(0), but it is highly sensitive to ambient air density (rho). The initial spin-rate (p) of the rifle bullet is V(0)/Tw, where Tw is rifling twist-rate in feet per turn. I mentioned that Sg is proportional to (p/V)^2. Right out of the muzzle, initial Sg is proportional to [(V(0)/Tw)/V(0)]^2 = [1/Tw]^2, which means initial Sg is determined by rifling twist-rate (Tw) instead of muzzle velocity V(0). While this is explicitly true, the aerodynamic overturning moment coefficient, itself, does depend upon the bullet's airspeed in Mach numbers, so higher muzzle speeds do produce relatively lower overturning moments and slightly higher initial Sg values.
Jim Boatright

 

[Greg Langelius *]

Jim, could you please indulge me?

Can you plot for me a range of velocities where typical LR bullets (i.e. 6.5mm/142SMK) enters transsonic at sea level and at 5000ft ASL.

I've been using a SWAG value of 1300fps.

Greg

 

[Jim Boatright]

The differences among high-drag slugs and very-low-drag (VLD) bullets in terms of the airspeed boundary between subsonic and transonic airflow regimes is about Mach 0.80 for blunt bullets and Mach 0.90 for "streamlined" bullets. This is about 100+ fps difference in airspeeds in most surface environments.

In slowing from supersonic into the transonic, the shape of the bullet is still a major factor. I use the reciprocals of the subsonic transition points(i.e., about Mach 1.25 and 1.11 respectively) as a working approximation, although this is certainly not definitive. Expressed in airspeeds in a standard sea-level ICAO atmosphere (Mach 1.0 = a = 1116.45 fps), these upper transonic transition speeds would be about 1395 fps (worst case slugs) or 1228 fps (best case VLD rifle bullets). I use 1200 fps as the minimum supersonic airspeed in a sea-level ICAO atmosphere for my Ultra-Low-Drag (ULD) turned copper bullets having a drag form factor (i) of just 69 percent relative to the G7 Reference Projectile. The larger caliber ULD bullets (375 and up) could thus remain supersonic at reasonable launch speeds to a range of two miles at 6500 feet altitude in an ICAO atmosphere at Raton.

Ambient atmospheric conditions determine the "speed of sound" (a, in fps). The airspeed a varies most significantly with ambient air temperature (T). For air temperature in degrees Fahrenheit, a = 49.0223*SQRT[T + 459.67]. The speed of sound (a) does increase slightly with increases in relative humidity and barometric air pressure (and decreases with reduced air pressure), but the actual physics is too complicated to put here. [A nomograph calculation has been used.] For example, the relative humidity correction requires knowing the partial pressure of water vapor in fully saturated ambient air, which is a nonlinear function of both air temperature and absolute air pressure. Air is a mixture of gasses, each with its own particular partial pressure in any given air sample. The speed of sound (a) also varies with CO2 and other gas concentration variations in the air. The variations in speed of sound (a) usually cited as a tabular function of altitude (h) relative to mean sea level are mostly based upon the air temperatures (T) expected at altitude in some particular atmosphere model. Ballisticians use the aviation industry's ICAO standard atmosphere model. So, you should just use the temperature (T) correction given above for a, and call it close enough for ballistics work. Many will recognize that the variation of a with absolute temperature (in degrees Rankine or Kelvin) comes from the Ideal Gas Law, which is itself only a good working approximation.

I hope this helps.

Jim Boatright