308 sierra speed and coheicient

[iagbarrb]

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

 

[Laphroaig]

I doubt that the ballistic coefficient has anything to do with the group size. Increasing the velocity to be greater than 2800 fps will result in a very slightly flatter trajectory for the distance the bullet remains over 2800.

Considering a 175 grain projectile in a 7.62 x 51 you'd be hard pressed to get that velocity anyway.

Laphroaig

 

[Bart B.]

When Sierra introduced their 30 caliber 168-gr. match bullet with ballistic coefficients of .468 and below, they were shot out at 2200 to 2300 fps from .308 Win cases in 300 metre free rifles in international competition (Olympics) winning matches and setting records.

Popular match winning and record setting loads with that same cartridge and bullet in traditional high power matches used reduced loads at 200 and 300 yards. Bullets left about 2400 fps that tested 1/2 MOA or better at those ranges.

Accuracy is when trajectories are repeatable. I would not change your load if it tests as you say.

 

[627PCFan]

I have always found that the best groups I get regardless of caliber and cartridge is at or around minimum or even below mins. I believe its harmonics related.

 

[iagbarrb]

Thank you very much for the posts. Seems to be I have to stay with the load I have. And it is true, changing load will change the harmonics.
0.56 MOA at 300 meter will keep the impacts in the center of the bullseye that has 2 inches diameter, but the smallest change in wind or temperature will place one impact out of the winning area. Do you have any suggestions to get 0,3 MOA ot tighter groups?
Thanks again.

 

[ArchAngelCD]

In reality 300 meters install that far for a match. I see no reason to use a 175gr bullet at those distances or even a 168gr bullet for that matter. I feel you will get better results with a 155gr match bullet from Sierra. It will require less powder and generate less recoil aiding in accurate shooting. I think the 155gr match bullet will be better suited for a .308 Win round if 300 meter shooting. In any event, I would at least drop from a 175gr bullet to a 168gr bullet.

I might be wrong so I'm hoping some of the other competition shooters will verify or correct my suggestion.

 

[farmerboy78]

I use the 175gr nosler custom competition and I'm loading 44.5gr of varget in lapua palma brass and I'm getting right at 2770 fps from a 24" barrel. I'm shooting it from a remington 5r milspec and it's a solid 1/2 moa load at 300 meters... but I have bought some of the 155gr palma bullets to try and I believe that bullet may be beneficial for recoil and a flat trajectory.