For accuracy, well over supersonic or merely supersonic?

[IMtheNRA]

I'm starting to develop a .308 round for 1,000 yard plinking. I'm working up my loads and have not yet reached published maximum recipes. It would be great to stay under max, if I can.

So far, several chrono and calculator results suggest bullets will remain supersonic at 1,000 yards, but only barely so, retaining a velocity of 1,200 to 1,276, depending on which ballistic calculator I use.

Speed of sound for that same set of environmental variables and altitude is 1,143 according to Strelok calculator.

So I was wondering if I need to exceed the speed of sound by a certain margin, or is just barely supersonic good enough to avoid transonic destabilization.

 

[jwrowland77]

As long as it stays accurate, you should be good. What bullet you using? Powder? Just wondering. I run my .308 out to a grand also.

 

[IMtheNRA]

Surprisingly, I'm getting good velocity from my tiny seventeen inch AR with a Berger .308 230-gr Match Hybrid Target (part #30428). Using load data on pages 694-695 of the Berger manual 1st ed:

Five round sample loaded with Varget 35.9 grains (book max is 36.4) produced an average velocity of 1,971 fps.

Five rounds loaded with Viht-150 36.5 grains (book max is 37.8) gave me 1,969 fps.

If I need more velocity for 1,000 yards, I have some room with both, especially with Viht-150.

I shoot at high altitude, from 5,000 to 10,000 feet, so the bullets can remain supersonic around here even when fired from an itty-bitty semi-auto

I'll check 1,000 yard accuracy soon enough, but I don't want to go banging away at the 1,000 yard line if the bullets are going to just tumble into the target and all around it. That's why I would like to get some insights into the relationship between speed of sound and minimum acceptable bullet stability at the intended target distance.

 

[jwrowland77]

Some bullets can make the transition well, some not so much, from super through trans to sub. That's the biggest problem with 168's. I'll run some numbers in my Ballistic AE app and see if they're hitting that's transonic stage.

I would love to be shooting at 5000 feet up. Lol. I'm shooting at 350 so I'm having to push mine a bit more. I'm pushing my 178's at around 2800fps, which helps me stay super at 1000 yards.

 

[jwrowland77]

Based on my app, at 5000 feet using 1970fps, you'll stay super out to 1050yd. Anywhere higher, it'll be further out. You should be good to go. I did 50% humidity at 40 degrees.

 

[Robert101]

Take a look at your impact hole. Is the bullet stable with a good solid round hole? If so then your should be good. This is why I use paper targets mounted on cardboard. Just a thought.

 

[Coltdriver]

That is interesting about the altitude and the humidity. In Colorado it is not unusual to see humidity in the sub 15% range, especially as you get up to 7 or 8 thousand feet.

 

[Toprudder]

http://bulletin.accurateshooter.com/2015/10/the-transonic-zone-what-happens-to-bullet-stability-and-bc/

 

[High Plains Drifter]

JBM Ballistics

http://www.jbmballistics.com/

You'll notice that bullet BC will effect accuracy downrange in that staying supersonic out beyond 1000 is a good thing.

Loaded to 2750 fps or 2800 fps with a Sierra 155 Palma Match King, or 155 Tipped Match King will keep the bullet supersonic out to 1100-1200 yards, as both bullets have BCs of .509 and .514 respectively.

Accuracy will degrade after the bullet slows to the trans sonic range because the aerodynamic forces in that speed range will usually make the bullet start to wobble changing the trajectory.

Play with JBMs online calculators, and then you can make some range cards there as well.

Happy shooting!