My 6.5 PRC is modified a bit, the throat is long, my barrel is 31" and my coal is 3.082 ref. CBTO is 2.400 on my Hornady comparator with Berger 156 EOL's, so this may not apply to most, but I am trying N570 first. I talked to Berger tech, they told me the top velocity with N570 was with max 64.1 grains, 26" barrel and coal of 2.955" at 3029 fps. I worked up from their recommended start of 57.5 at 2680 fps. I found that 64 grains completely filled the ADG case, so I stopped at 62.5 grains. Velocities were:
57.5 @ 2933, 58.5 @ 2974, 59.5 @ 3037, 60.5 @ 3094, 61.5 @ 3148, 62.5 @ 3183.
I then loaded 63.5 and got 32.43 fps. As I mentioned 64.0 filled my case, but I seated the bullet to depth, compressing the charge more than I was comfortable with. I did fire it without any problems, but the velocity went down to 3237 fps. I have not ever compressed a load like this before. Thanks to the forums I found out that a drop tube helps for this, so I made one and got the powder to settle to a decent level. I loaded 10 rounds to tryout this weekend, I am attaching a photo of the drop tube I made, maybe it will help someone.