Since last time, these changes were made:
-- I backed off the case sizing die 1/2 turn (0.035"). [That was overkill, 1/6 turn would have been enough to correct the headspace.]
-- sized cases then protruded 0.0055" from the end of the barrel, pretty close to the calculated 0.007" ideal.
-- I changed the case trim-to length from 1.747" to 1.745"
After thinking about it some more, I doubt if the excessively-sized cases had been jamming into the throat hard enough to distort the bullet, because that would require force and no such force would be pushing on the case. Worst case, the excessively-sized case would have butted up against the throat entrance similar to a 45acp round. The main problem with the excessively-sized cases would be excess headspace leading to weak ignition.
With the changes made to the cases, I re-tested two of the same loads that I tested previously. Here are the before-and-after targets.
Velocity increased 84 fps with the AC4350 load and 41 fps with the N135 load, reinforcing the theory that ignition had been weak with the excessively-sized cases.
Accuracy was worse with the AC4350 load, but better with the N135 load. As always, one bad group is enough to prove that a load is bad, but one good group is not enough to prove that a load is good, so I want to repeat the N135 results at least one more time before I declare a victory.
I'm going to give up on AC4350 because it just doesn't burn well enough at these moderate pressures (44,000 psi). IMR4350 seemed to burn a little better, but I am nearly out of IMR4350.
Quickload has not been predicting velocities accurately with this particular lot of N135. QL predicted 2174 fps and 36,000 psi with today's 33.2 gr. charge, but today it actually produced 2310 fps. Since the relationship between velocity and pressure is fairly reliable, I would estimate that today's load generated 44,000 psi. That may be too much pressure for optimum accuracy, but I'll worry about that later. Next Time:
-- see if the N135 accuracy is repeatable now that the case sizing issue has been sorted out.If the N135 accuracy is repeatable:
-- if N135 can average 1 MOA at 2300 fps, then that will become my control load and I want to move on to testing bullet designs.
-- I may need to lower velocity & pressure if I want to do better than 1 MOA, but that can wait.
-- since 33.2 gr. N135 occupies only 93% of the available powder space, it might be worth trying the thicker Lake City cases.
But first let's see if the N135 accuracy is repeatable.