09-05-2013, 03:35 AM
26.5grs or AA2520 should be a safe starting load, but book values are showing 24.9gr for the Hornady 120gr GMX solid as a low starting load. I personally would want enough of a pressure curve saturated with data points so I could see what was going on with the load and my chamber. I would run 3 batches of pressure/accuracy ladders, starting with 26.5grs at 2.240 COL.
26.5
26.8
27.1
27.4
27.7
28.0
28.3
28.6
28.9
29.1 (Hornady lists this as the max load with the 120gr GMX solid)
Watch the chrono as your #1 focus for erratic velocity changes outside of the predictable pressure curve. You should see changes with each .3grs at ~25-40fps. Once you see significant increases, or even an unexplained major drop, record that, and measure case head dimensions above the extractor groove. If you see any major flattening of the primer, or cratering, you know pressure is way too high. When you run the other 2 batches, you should start to see some consistent trends.
You can also do round robin targets at distance to find the accuracy nodes without having to color bullets. You basically have an aiming point for each charge, and the batches will add up at each point until there are 3 holes per aiming point. You can mark the aiming points with the charge weight in big letters above, or assign each charge weight a number and annotate that next to your load data.
26.5
26.8
27.1
27.4
27.7
28.0
28.3
28.6
28.9
29.1 (Hornady lists this as the max load with the 120gr GMX solid)
Watch the chrono as your #1 focus for erratic velocity changes outside of the predictable pressure curve. You should see changes with each .3grs at ~25-40fps. Once you see significant increases, or even an unexplained major drop, record that, and measure case head dimensions above the extractor groove. If you see any major flattening of the primer, or cratering, you know pressure is way too high. When you run the other 2 batches, you should start to see some consistent trends.
You can also do round robin targets at distance to find the accuracy nodes without having to color bullets. You basically have an aiming point for each charge, and the batches will add up at each point until there are 3 holes per aiming point. You can mark the aiming points with the charge weight in big letters above, or assign each charge weight a number and annotate that next to your load data.