Keeping a Sierra .308 168 HPMK Supersonic at 1 Mile?

Art Eatman says elevation has an effect on the speed of sound. Thinner air = slower speed.

I disagree.

The speed of sound changes clearly with temperature, a little bit with humidity − but not with air pressure (atmospheric pressure). If sound speed did change with elevation, then music would sound off its original key if played at a different elevation it was recorded at.
Click to expand...


NASA seems to think it does.

http://www.grc.nasa.gov/WWW/k-12/airplane/sound.html

Granted, it isn't much, according to their calculator:

1116 FPS @ sea level
1076 FPS @ 10K feet

That calculator doesn't have an input for temp, which has a much greater impact.
 
going stupid

The first 1000 yd F-T/R match I ever went to (I've been to more 600 XTC types, but still a rookie irregardless), two of our bunch showed up with 168 factory match ammo, I think Black Hills, and had at it at 1000. Both guys left after the first relay. From their rifles, and barrel lengths, that ammo, supposedly grouped very well at 100, but by 1000 the slugs had "gone stupid" (wish I had thought of that phrase).

Just kind of pooped past at 1000.

I have never heard of "No 168's" at 600 for an F or F-T/R match either.
 
No offense to audio engineers, but I will defer to the rocket scientists on this one.

In reality, whether it does or not isn't all that relevant, as it is only a 40 FPS difference (according to NASA), and the effect of altitude/air density on drag plays a much bigger role in LR target shooting than the speed of sound.

For example, my 6.5-06 shooting 140 SMKs at ~2900 fps, on a 59ºF day according to JBM:

At sea level, is still traveling 1406 FPS at 1000 yards., and goes subsonic at around 1350 yards.
At 5000ft elevation, the bullet is traveling 1605 FPS at 1000 yards, and goes subsonic at around 1600 yards.

In the same conditions, a .308 168MK @ 2700 FPS:

At sea level is traveling 1105FPS at 1000 yards, and is subsonic*.
At 5000 Ft, is traveling 1273 FPS at 1000 Yards and does not go subsonic until 1200 yards.

*According to JBM, and the sound engineer calculator linked above agrees, the speed of sound at 59ºF is 1116.4 FPS. It is also interesting to note, that JBM does not adjust the speed of sound with altitude.
 
Blackops mentions:
168s are usually the standard for 600yd competitive matches like F-class from what information i gathered.
Click to expand...
When the .308 Win. was "the" cartridge used in NRA high power rifle competition, the favorite bullet for best accuracy and scores was the Sierra 190 HPMK in both bolt guns and 7.62 Garands. Sometimes, Sierra's 200 HPMK was used in bolt guns. I watched Corky Tyson put twenty 200's into about 4 inches at 600 yards using aperture sights setting the record for that prone course of fire. David Tubb shot Sierra's 250 HPMK's in a 1:8 twist barrel leaving at 2150 fps and shot very accurate indeed.
 
I honestly want a 30-338 lapua. I know brass would be outrageously expensive. But i would love to be throwing 240gr SMKs out of it. I guess by that weight in .30 cal you might as well just go .338. I want to try some 220 smks or 240 smks in my .300 wby though.
 
The reason that calculators (and tables) show a change in the speed of sound with altitude is that a certain decrease in temperature as altitude increases is assumed.

The reason that altitude doesn't affect the speed of sound is because the density and pressure offset each other. The air pressure goes down, but the density goes up. And that's because as one ascends in a column of air, as the temperature decreases, the density increases, while the amount of air in the column decreases, lowering the pressure. This holds true up through the stratosphere, at which point ozone heating comes into play and the speed of sound goes up...and eventually, the air goes away.

There are some meteorological exceptions to the rule, such as with high altitude winds or around thunderstorms, where the air does not behave uniformly, but who's out shooting in a thunderstorm?

By the way, we should be happy that air behaves the way that it does. In other mediums (like water, for instance), different frequencies have different speeds of sound.

As far as keeping a bullet supersonic at a mile, I have no idea...I can barely see the target at 200 yards.
 
You must log in or register to reply here.
Back
Top