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"Next Step : Once fired, Choose the recipe with the best grouping , eg. 68.5g , and load up 3x rounds of 68.1g, 68.3g, 68.4g, 68.5g, 68.7g, 68.8g ?"

Yes you could load 5 of each of those powder...

You can still use the pulled projectiles. If you are using a collet type puller just don't clamp down on them too hard.

Maybe post a picture of the .300 projectiles, Id think they would be ok and...

The method of shooting a group with each powder charge and checking for groups is good. Just don't start that high because it will probably be time & effort wasted.

After determining the max...

I would not start that high. Its going to suck if you get to the range and find your start load is over pressure or close to, and you cant shoot any of the other 45rounds...

This is what I would...