.308 sierra coheficient and speed

I

iagbarrb

New member
Dear Friends,
I have started competing in 300 meter Benchrest with .308 Hunter.
Reloaded some with sierra 175 gn Sierra HPBT Match King and got a speed from 2300 fps. With those I had very nice groups 15 shoots in a 2 inch diameter radius at 300m. About 0.56 MOA at 300 meter, light wind sunny day 56-60 Farenheit 650 milibar (Hight altitude)SSG 69 Steyr rifle. Just changing bullet of NATO FN Belgian 7.62 ammo and decreasing the charge of the powder from the original 44gn to 40 gn. (No idea which powder uses FN for its NATO ammo, nor the primers.)

According to the data published by sierra, the coheficient is .505 over 2800 fps, and .496 below 2800

Wonder if increasing the speed getting the better coheficient I might get better groups.

I know I have to test it by myself, but would like to know if some of you has experience with it and can share the results.

Thank you
 
Higher velocity + good BC = less wind drift.
Not necessarily smaller groups in calm conditions. But the wind does blow.

The 173 gr M1 bullet at 2300 fps was a common 300 metre load in the 1930s.
 
Most benchrest shooters go for the heaviest bullet they can shoot the fastest with the slowest possible twist.

Since you are in the hunter category, why not try a 190gr BTHP? You get a better BC at all speeds, and starting charges of IMR4064 will be over 2,300 fps and over 2,500 fps at max charge.

Jimro
 
Iagbarrb,

The difference between 0.505 and 0.495 BC at 300m in a 20 km/hr (5.55 m/s) cross-wind is 0.08 moa at 300 yards. I would ignore that factor, as the difference is so small. However, if you fired at 2500 fps, then the time of flight for the bullet to go 300 yards is reduced enough and the velocity loss not greater enough that you will save 0.36 moa of wind deflection in that same wind. At different wind speeds the effect is roughly proportional to wind velocity.
 
Thanks for the posts.
The speed increase to 2500 looks very interesting by saving 0.36 MOA I will absolutely try this. 0.56 MOA groups in a 2 inch diameter winning area are in a borderline position where a small variation of temperature in bullet or atmosphere might place an impact or two out o the center. Lets see what harmonics say with a load increase.

I would love to try a slower twist, but in my country it is not so easy to get a second rifle. Same applies to projectiles. As far as I understand, the steyr mannlicher ssg69 P1 has a 1:12 twist.

In my next visit to the US I will buy the 190 gn and try how it works. Powder, they do not allow powder or primers in the plane. So I still will have to use military powder from Belgian or American ammo that has to be disassembled. but even this ammo is getting out of stock.

The powder I can purchase here is Brazilian CBC 102. This powder just su*&$.

Anyway, any comment that will help getting tighter groups will be appreciated. Goal is national championship.
 
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