We can look at the following results to make our determinations as to what seating depth has delivered the best accuracy. Target #1 was shot at a 2.130” COAL, and the group measured 0.432” inches. Target #2 was shot with the bullet seated 0.005” deeper, at a 2.125” COAL. This group measured 0.513” inches. Target #3 was shot with the bullet seated another 0.005” deeper, at a 2.120” COAL. This group measured 0.565” inches. All targets were shot at 100 yards, with the Berger 52 grain Match Varmint bullet, and no other load changes were made other than bullet seating depth.
As you can see, we have been able to show a noticeable decrease in the group size. We would be looking at continuing to work from the SAAMI 2.130” COAL, since it delivered the smallest group fired during this test. This rifle would be considered to be capable of 1/2 inch accuracy, and we now have a round that will not only feed from the magazine, but is shooting very good groups. This had been accomplished with the bullet being jumped 0.056” off of the lands!
We are using the bullet seating depth to tune for an optimum accuracy node in the barrels vibration cycle. The goal is to have all bullets to exit the barrel in the same location in its vibration cycle! Once you accomplish this, you will be well on your way to keep your loads, in tune, for maximum accuracy for the life of your barrel!
PS: We would like to thank the folks at the Indiana DNR, Roush Lake Shooting Range facilities, for the use of their range facilities for this testing.