I have just started with the .300 with a 22” barrel. I am using my Thunderbeast Ultra 5 suppressor with it and I’m hunting the EC with it this month. I had all kinds of trouble and bought like 4 or more different loads. I was frustrated. What I finally figured out is that it gets hot and loses accuracy. Just like you I had poor groups and the occasional flyer. Frustrating!
Then I finally planned to just shoot a three shot group and then go back to work (yes I have two shooting ranges at my ranch so I can shoot anytime). I am wanting to hunt with Barnes 180g but also bought and had good accuracy with Nosler Accubond 180g.
With a cool barrel I am just about putting them in the same hole. 1” group at 100 yards easily. Problem solved! Thank goodness as I leave the 24th.
Best of luck.
Philip