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Vibe
11-15-2008, 08:47 AM
Dang. I almost hate it when this happens. Husker made a pretty good reply to a post I made in response to a comment by Lynn on the Stopped Muzzle mega thread. I was lucky enough to have gotten his reply in an e-mail notification before the replies were "cleaned" from Bills thread.

My comment had to do with the reduction in wind drift being related to initial yaw of the bullet.
Huskers post was one that makes a very valid point and I did not want it to get lost so quickly.



While Vibe’s explanation may very well be correct…it appears the simplest solution to achieving a reduction in wind drift with projectiles operating inside the Mach 1 envelope is to simply lower the velocity of said projectile.

The research done by Dr Robert McCoy and Professor Art Pejsa shows this Mach 1 envelope extends a couple of hundred ft/sec above and below Mach 1 velocity. It is not necessary for a rimfire bullet to be above the speed of sound to be affected by anomalous behavior from flight in this envelope!

Here is some data output from Professor Pejsa’s ballistic engine showing muzzle velocity followed by the wind drift figure for a 10mph wind at 50 yds with a BC of .160: (The BC of .160 is an average derived from the testing Dr Robert McCoy did at the Aberdeen Proving Grounds on a Eley semi-round nose bullet but is pretty much a meaningless number because any change in velocity, distance, or deformation of a soft lead bullet will drastically affect how BC is calculated on projectiles operating in the Mach 1 area.)

1,085-.78”
1,075-.75”
1,065-.74”
1,055-.74”
1,045-.74”
1,035-.50” (Much more than a bullet width with a 10 ft/sec change)

If any of a dozen variables is changed, this bump in an otherwise smooth progression can and will occur at different distances or velocities.

I contacted Professor Pejsa during my research on rimfire ballistics and he assured me the output from his software was correct but I wasn’t given a detailed explanation as to why.
I’ve read one of Professor’s Pejsa’s books and “attempted” to wade thru some of Dr McCoy’s math but the majority of calculations are beyond my abilities. Predicting the aerodynamic flight properties of projectiles in the Mach 1 envelope may be “the” most daunting of tasks in all of ballistics.

Someone like Bryan Litz could contribute greatly but he’s more interested in missile guidance and supersonic centerfire projectiles.

Landy

HuskerP7M8
11-15-2008, 09:07 AM
Thanks Vibe

I post only rarely and had forgotten about that thread being purged on a regular basis.

I just thought there might be a few people interested in the bizarre behavior of rimfire ballistics and another explanation for what Mr Calfee may be observing.

Landy

Rich In Kansas
11-15-2008, 09:12 AM
This is a mixed bag. I have an old NRA fact book somewhere that has an explanation. http://members.cox.net/benchrest/Rimfire_notes.html#Wind
I have it copied here. Anyhow down to 950 fps gives even less wind drift I used an online external balistics program. BUT accuracy goes to pot. Federal found that out when they came out with their ultramatch back in the 90's. It was very accurate but drifted way to much. They got lots of complaints so they came out with Ultramatch "B" that gave up a bit in accuracy but drifted less in the wind. So the accuracy needed in any given gun is a mixture of good accuracy and less wind drift. It would seem to me that the projectile being one size fits all might make the compromise worse. What if one projectile for 1020 another for 1030 etc. I guess if the ammo makers did that, we would think the new Lapua X-Act was cheap. Rich

HuskerP7M8
11-15-2008, 11:09 AM
Rich,

Less wind drift with reduced velocity is common knowledge in smallbore.

My post was made to point out the near impossibility of predicting ballistics in this velocity range because a bullet’s drag coefficient increases at an almost logarithmic scale instead of in an easily calculable progression as it enters/leaves this envelope. Because of this you can observe dramatic differences in wind drift with minuscule changes in the variables and could be led to false conclusions when trying to make assumptions based on a testing session in the wind.

As regards the many external ballistics programs available both online and as stand alone packages…most, but not all, are aware of this strange behavior and use different methods to calculate for this phenomenon and I don’t think any of them will give exact/identical solutions with duplicate variables for input. Many of them are simply not interested in accuracy for subsonic projectiles…they only care about supersonic centerfire.

BTW My calculations show the perfect velocity at less than yours and it’s distance specific, but as you suggested…accuracy deterioration overshadows any wind drift advantage.

Landy

Rich In Kansas
11-15-2008, 02:18 PM
Landy, I didn't think I was disagreeing with you. As far as the 950 fps I mentioned, I don't remember exactly, this was 5 to 10 years ago I did this, so you are probably right. The point I was trying to make is that minimum bullet drift is obtained at much lower velocities than we have available, but to load ammo that slow accuracy will suffer, so a compromise is required.

Vibe
11-16-2008, 03:59 PM
One card was 2050 the other was 2075..........the temp was 39 degrees as I left the range.......still raining and still windy...

Where am I at? Ain't sure......I know what I'd like to say I believe, but I won't just yet.....


Looks like we will probably go through the "Eley EPS keyholes in cold weather from 17+" twist - no it doesn't" discussion again this year.