The .243s I’ve had, including a Ruger American, have liked anything loaded with the following bullets:
Nosler 70 Ballistic Tip
Sierra 85 BTHP
Hornady 87 V-Max
Winchester Supreme varmint ammo (black box) using a 55 grain Combined Technology bullet makes little groups but I don’t think is made anymore.
Hornady varmint ammo using the 58 grain V-Max has done great also. I think it’s currently sold under the Superperformance line.