Fine-tuning load AND rifle.
I've been developing a long range hunting load for my Tikka 595 in .260 Rem. This rifle has a 26" Pac-Nor sporter weight barrel with a Muzzle diameter of 0.650 inch. The barrel was a 1 in 8" twist and polygonal rifling. The action is on its factory synthetic stock, with the screws tightened to 7 N-M (62 lbs.-in.).
Following all the published recommendations posted in this and other forums and websites, the most accurate load I could assemble using available weight-sorted and prepped Remington brass was the generally published load of 47.0 grains of H4831 SC, Federal 210 M primers and Hornady 129 gr. SST bullets (I intend to shoot at animals at no more than 750 yards). The bullets were seated to engage the rifling.
All loads were tested at 300 METERS (327 yards) and this particular OPTIMUM ACCURACY load produced a 5-shot group with a 2.54 inch spread. (about 0.67 MOA). Average Velocity was 2935 fps with a Std. Dev. of 7 fps and an extreme spread of 21 fps.
Having read a few articles on the effect of tuning the action screw torque to improve accuracy, I carried-out a little experiment yesterday at Tucson Rifle Club.
I assembled 12 rounds using this load and shot 3-shot groups at 300 meters, varying the amount of torque in the action screws (I use a Gehmann torque wrench) in 1/2 N-M increments, starting with 5N-M. Results were as follows:
5.0 NM - 5.2" group size. Spread mostly vertical
5.5 NM - 4.2" Same spread type
6.0 NM - 3.5" Spread mostly horizontal
6.5 NM - 1.45" - Spread mostly horizontal
7.0 NM - 2.65" - Spread equally vertical and horizontal
All groups were shot out of a "cold" or ambient temp. barrel.
Finally, I a similar load of 47.0 grs. of H4831 SC and Nosler 130 Accubond bullets in Nosler prepped and neck-turned virgin brass produced the following results with the action screws tightened to 6.5 NM:
Avg. Vel. = 2857 fps
Std. Dev. = 1
Extreme Spread = 2 fps.
Group Size = 3.5 inches - Spread equally along vertical and horizontal
The group size is not acceptable, so the action torque experiment will be repeated at a later date, from 6.0 through 7.5 N-M in 1/2 N-M increments.
I don't know if the fact that I was using virgin brass had any effect on accuracy, but for consistency's sake, I will continue using the remaining 41 virgin Nosler cases for testing.
I am curious to know if anyone out-there has tried this and if so, what the results were.
Joaquin B. in AZ