Actually "shooting off open pickup door" tells me very little about what the gun is capable of. It does say a little about what the shooter is capable of, but you shoulda stopped with the 100 yard group. It's pretty good for off the pickup door. On the other hand the size of the 300 yard group (
way bigger than 3X the 100 yard group) tells me that maybe the 100 yard group was a fluke?
Get a good solid bench with sandbags fore & aft (or a bipod up front). Prone will work also, with a bipod & rear bag. Then you can be solid enough and consistent enough to truly see what you and your new rifle are capable of. Then you can start trying different ammo, bullets, powder etc to fine tune things. Change one variable at a time or you'll end up chasing your tail in circles.
It can be a fun project. Good luck!