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 May 23, 2007, 06:20 PM #1 Glennster Senior Member   Join Date: July 31, 2005 Location: Northern Indiana Posts: 226 FPS = Super Sonic Thanks for your help with the BC on the .223 75 gr. Can you tell me how many feet per second equals Super Sonic. I'm trying to figure out when this load will go from super sonic to sub sonic. THANK YOU Gentlemen !!!
 May 23, 2007, 06:31 PM #2 rwilson452 Senior Member   Join Date: June 10, 2004 Location: Tioga co. PA Posts: 2,493 speed of sound At 75 degrees F and at sea level pressure 50% humidity the speed of sound is about 1100 FPS.
 May 23, 2007, 08:54 PM #3 Glennster Senior Member   Join Date: July 31, 2005 Location: Northern Indiana Posts: 226 Thank you Sir ! ! !
 May 23, 2007, 10:21 PM #4 rwilson452 Senior Member   Join Date: June 10, 2004 Location: Tioga co. PA Posts: 2,493 more onthe speed of sound The speed of sound is effected by air density. The more dense the air the higher the speed of sound. Hence teperature, barametric pressure and humidity change the actual speed of sound. As an exanple in sea water at sea level and 39 degrees the value is 4800 fps but raise the temperature to 78 and the it goes up to over 5000 fps air is the same way. it's complicated enough that they make tables to predict it. If you are not too high in altitude and in shirt sleeve weather 1100 fps should suit your needs. assuming your using a center fire rifle by the time the bullet slows to that point velocity is falling rapidly.
 May 24, 2007, 12:14 AM #5 mrawesome22 Senior Member   Join Date: November 9, 2005 Location: Ohio, Appalachia's foothills. Posts: 3,779 Wilson is right on. Humidity plays a HUGE part in the speed of sound. And humidity is constantly changing at different elevations and atmospheric conditions. Anyway, higher humidity, sound travels faster. Lower humidity, sound travels slower. And sound degrades much less through water. Last edited by mrawesome22; May 24, 2007 at 12:24 AM. Reason: Spelling
 May 24, 2007, 12:23 AM #6 rwilson452 Senior Member   Join Date: June 10, 2004 Location: Tioga co. PA Posts: 2,493 Perhaps the fact that some 30 years ago when I was in the navy, one of my specialties was as a physical acoustical anaylst. It might explain why I can still pull this stuff off the top of my head.
 May 24, 2007, 06:32 AM #7 Glennster Senior Member   Join Date: July 31, 2005 Location: Northern Indiana Posts: 226 You guys are great, I appreciate the help ! ! ! ! The reason for the question is that I've got a .223 (75 gr. Hornady Match , 1:7.7 twist, 2800 FPS) and a .308 (168 Berger VLD, 1:13 Twist, 2620 FPS) that I'm trying to go 1,000 yards with. As I plot out the BC, FPS, ect., I wanted to figure out when the load would go below super sonic. It looks like I'm real good out to 800 yards, 900 is possible, 1,000 is a bit spooky...... : - )
 May 24, 2007, 06:47 AM #8 Jim Watson Senior Member   Join Date: October 25, 2001 Location: Alabama Posts: 13,322 I have done some figuring and shooting in that vein. On Sierra Infinity, default mets, it looks like the 75 gr Hornady Match will be marginal at 1000. The 75 gr A-Max has a higher BC and will give you a cushion; if it groups in your rifle. The .308 168 VLD is fine. Me? I have a 6.5 twist .223 for 90 grain bullets and the JLK and Berger VLDs are very good but the 90 gr SMK is overloaded at high enough velocity for 1000 and the 75 gr A-Max is overspun; neither can be counted on to make it all the way to the target. .308 155 gr Scenar is looking good.
 May 24, 2007, 03:34 PM #9 30Cal Senior Member   Join Date: December 3, 2002 Posts: 1,194 You want to avoid the trans-sonic region entirely (not just the exact speed of sound). I'd stay above mach 1.2 (~1300fps) to avoid the associated affects of increased drag and turbulence. This will also give you margin as temp, humidity, & pressure changes. Ty
 May 25, 2007, 02:42 AM #10 Bullet94 Senior Member   Join Date: September 8, 2004 Location: Kansas Posts: 723 Glennster You might look here - http://www.eskimo.com/~jbm/calculati...raj_basic.html __________________ PRO-SECOND AMENDMENT - Live Free or Die
 May 25, 2007, 04:02 PM #11 Glennster Senior Member   Join Date: July 31, 2005 Location: Northern Indiana Posts: 226 Thank you, I'll use that site !

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