transonic velocity for 308 win?

[rap36case]

I have read books and articles about transonic velocity for long range shooting. As far as I can conclude, Mach 1.2 translates to 1339fps and is a good guideline for transonic at 1000 yards.

I am curious if this is correct or, if there are other reasoned opinions. I want to use this as a guideline when developing a load for my 308win. I shoot a match select 26" with 1 to 10 twist.

 

[M-61]

Altitude.

 

[Dgd6mm]

You are correct at or above Mach 1.2 at 1000 yards to stay supersonic. Every once in awhile there will be someone shooting a load that goes transonic before 1000 yards while you are pulling targets. You can hear it, and it shows on target. What bullet do you plan on shooting?

 

[Keith Glasscock]

Altitude.

How long it takes to get down to the transonic range is related to absolute air density (altitude being part of it), but mach number is related to temperature.

So, the velocity in fps where the transonic range begins will only be measurably affected by air temperature.

 

[rap36case]

I have read some of Bryan Litz excellent books on ballistics. I know that temperature and altitude affect velocity and point of impact. Here is the load:

Laupua 308win trim to 2.010, wall thickness 0.0115, Neck 0.329, using high precision dies found in "reloading for competition"
CCI BR2
Varget 41.5 to give terminal velocity at 1000 yards of 1350fps from a Shilen 26" with 1 to 10
BERGER CALCULATOR on their web at 80F 1mph from 3 O'clock at 700ft
Berger 185 OTM seated to 0.018 jump

Note that at sea level and -10F this does not work and I would not be shooting. This is different from 76F at 5000ft. Hence the importance of labeling your loads and having a good meter such as Kestrel.

I am most likely to punch paper at 1000 yards in Texas, or hunt hogs at long distance. Hence not much change in altitude, and temperature range from 20F to 106F during the year.

I like the idea of using estimated velocity at 1000 yards as a guideline for powder charge. This means I can load the heavies with high G7 to get 1339fps at 1000. I plan to do this without modifying free-bore. This is my working hypothesis so far...