The 340 grain has the best looking group, including potential and size. When you initially posted the molded bullets I had the 370 grain as a long range contender, but it looks like it is hitting at a yaw (or the target was not square with the bench).
Shake and bake powder coating pretty easy .check out Elvis ammo on YouTube he has some excellent videos on the subject. I can powder coat 25 lbs of slugs at a time with this method.. I know with my .510 Texan when I order molds they need to be .512 or .513 in diameter then sized to .510 for good accuracy. When I started I ordered a mold at .510 diameter. It through s at .509 and not good for accuracy. Go a few thousands over your diameter and size down. My experiences with a few different molds.
Cut 3 coils from my bushbuck hammer spring making it 5/8" shorter than stock. I can now get a bell curve with a 4000 fill with the 2nd shot only 10 fps slower than the first shot with stock spring. Also not so much an air hog. Shot groups today at 60 yards with this spring and 3500 fills. Performing much better.