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[it is my understanding that] as a projectile pasts through the �sound barrier�, it is subjected to turbulence that is created. This is present both �up� and �down� through said barrier. The biggest problem (let�s say, most upset) will occur with a projectile which is, at that point, �unstable�. (from whatever cause)

The question would be: �In your opinion, is there an advantage to KEEPING the velocity either above or under the �sound barrier� (velocity) to avoid this phenomena?

Also, I was of the understanding that said �sound barrier� was dependent upon (1) temperature (2) altitude (3) humidity. But after retracing this, find computers, on line, that only use temperature for this calculation. Soeed of Sound Calculator
(or, am I confusing Mach Numbers and Sound Barrier?)

As to the last question, Mach number is a measure of the speed of sound, that speed being Mach 1. Mach numbers may represent speeds higher or lower than 1 and is usually represented to two decimal places, ie. Mach 1.02. In rough calculation, 1/100th of Mach represents 6 knots, or about 6.5 mph.

As to the rest, I discussed this earlier as you no doubt recall. First off, check the attachment, and sorry for the low quality but I think it will suffice.

Velocities represented are:
Upper left - 1135 fps
upper right 1255 fps
lower left 1604 fps
lower right 1636 fps

To clarify the question regarding turbulence if I can...it is not turbulence in the sense of bouncing around in the sky, but rather an issue of where the point of separation between laminar(smooth) and the turbulence that follows laminar separation. Despite the existence of the turbulent region along the bullet there is another consideration, that being boundry layer dynamics. It is an extremely thin layer of air that has very little movement and it is found betwixt bullet and other flows, be they laminar of turbulent.

If you can see it in the attached photo(Thanks to Lyman, hope I don't go to a hot place for that. <img src="/ubbthreads/images/graemlins/shocked.gif" alt="" />), there is a shock wave preceding each bullet by substantial margin, and exists as long as the bullet, or flowfield has supersonic characteristics. Bullets generally need to be at or above Mach 2 for the bullet nose and this shock wave to meld. It is not a mysterious wall, just a shock wave, the difference in air pressure on either side allowing the phenomona to be photographed. Here's the part I hope you can see...at the point where the ogive meets caliber diameter you will see another disturbance, possible a line or dart like feature. Airflow behind that point is generally turbulent, and the point of separation moves slightly fore or aft as a function of Mach number. This affects ALL bullets. You will also note turbulence behind the bullet base, represented by fuzzy swirls etc. BTW, the dark line is a wire that triggers the camera when it is broken.

The point I tried to make earlier is that Mach 1 is the point of highest drag, again, for ALL bullets. The bullets shot in BPCR have vulnerabilities, ie, defects that may be caused by acceleration forces, marginal twist rates, chamber pressures deforming bullets, slumping etc. Anything that detracts from a bullets stability, regardless of the reason will be most affected at this point. If you have the stability, you're good for speed I'd say, otherwise no. The key to understanding this is what lies behind our need for rifling in the first place. Conical bullets require gyroscopic stability because the center of aerodynamic forces is in front of the center of gravity. Kind of like trying to make a shuttlecock fly backwards.... Those aerodynamic forces are at their very greatest at Mach 1 as far as you fellas are concerned, their OVERTURNING Moments are greatest at that point, so that is when a marginal circumstance of gyroscopic stability is most vulnerable.

For purpose of calculation, temperature is used to determine Mach for a given velocity.



Oh yeah, how about them grease grooves? Mach 1.6

Dan, thanks for putting that up. I recall the discussion earlier (albeit, I am prone to occasional 'forgetfulness' <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />)

In Long Range BPCR, it is not uncommon at the extreme ranges for a bullet to be various stages of (un)-stability.

And, when shooting at the shorter 120-220 yard ranges, the thought crosses my mind that (possibly) putting the bullet into and then back out of hypersonic could create a little larger grouping. <img src="/ubbthreads/images/graemlins/confused.gif" alt="" />

I also shoot 6mm benchrest and just have a 'thing' for seeing how tight I can get a BPCR to group off the bench on my 120 yard range. Such is generally viewed by most BPCR people as 'silliness', but to me it is a challenge.

As I said, it appears to me that to negate any such effect by either keeping under supersonic to the target, or to keeping above it to the target is necessary.

An opinion only, but at shorter ranges I'd say subsonic from the start would be better. Less stress on the bullet, less drift, less recoil, less inches in the group. There is a point at longer distances when your target would be easier to hit if laying flat on the ground...assuming a slow speed launch of course. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> I do agree that you are better off staying on one side of the fence or the other if you can. Not because of turbulence, but rather the mix of larger overturning moments and imperfect bullets.

Interesting Dan,
I'm always curious as to how aerodynamics interplay in our bullets. Though my Jenny replica is only Mach .1, it is amazing what performance improvements happen with a bit of fairing.
As some here know, I think that a NASAII bullet that I have with small ridges in the nose, actually works better than an identical smooth nosed one- Boundry layer breakup??
As a practical matter, to fire a BPCR bullet over or under Mach 1 ALL the way to the target @ 1000 yds. is going to be tough to do with many attached problems. Hot light chicken loads seem to be reasonable though & many load them that way.
Maybe Dan T. is getting it right- grooveless bullet & wiping, but now he is back to mini-grooves. Maybe we need a mini-mini groove (ridges the length of the bullet??) <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />.. Just "Cranking" up as usual.... Dennis

Dennis; is there a picture of the bullet to which you speak available? (curious)
thanks

Idahjo,
The ridged NASAII photo is over on MSN BPCR under Photos- Pathfinder. Bernies molds are "cherry cut" & as a result are not perfectly smooth like a lathe cut mould. The first one that he cut had some chips in it that gave it slightly more pronounced ridges. For some reason, it always gave me better groups- it's what I shot in the Tryouts this year. Even though I had five bad wind call misses, it stayed in the higher scoring rings when I didn't have to adjust much for condition (I'm a lousy wind reader!).
Don't know that Dan ever posted his mini-groove or new concept bullet. ...Dennis

Got on BPCR.net; but can't find 'photos'... am I blind <img src="/ubbthreads/images/graemlins/confused.gif" alt="" /> <img src="/ubbthreads/images/graemlins/crazy.gif" alt="" />

Dennis' NASA II on left, Klaus' on right

The two NASA bullets used at the AZ LR match have the following specs:




NASA I (Klaus' by Mike Lewis):

Weigh: 545 grains

Length: 1.45�

Meplat: 0.24�

Ogive Radius: 5.2 calibers

Nose Length: 0.690�

Boreride Diameter: 0.450�

Boreride Length: 0.300�

Driving Band Length: 0.140�

Driving Band Diameter: 0.458�

Grease Groove Length: 0.200�

Grease Groove Depth: 0.020�

Base Band Length: 0.150�

Base Band Diameter: 0.458�






NASA II (Dennis' by Bernie Rowles):

Weigh: 543 grains

Length: 1.450�

Meplat: 0.23�

Ogive Radius: 7.8 calibers

Nose Length: 0.89�

Boreride Diameter: 0.450�

Boreride Length: 0.100�

Driving Band Length: 0.100�

Driving Band Diameter: 0.4570�

Grease Groove Length: 0.250�

Grease Groove Depth: 0.020�

Base Band Length: 0.100�

Base Band Diameter: 0.460�

Jones Creedmore - Lyman Postell - No groove outside lubed

They're buried in Theodore's tech articles...may not be blind, just looking in the wrong places?

LOOK?: NASA Bullets /Tech Articles/ Dan Theodore report on NASA bullet in AZ