Sometimes it's the little things that are easy to overlook.

I'm glad you found what may be the problem. Let us know how it shoots after you tighten the action screws and try it again.

I've found the Hornady 300 gr RN Interlocks to be very accurate in three different rifles; a Ruger No. 1 that I no longer own, my Ruger M77 Magnum and a friend's Ruger M77 Magnum. All three rifles shot the above listed 68.0 gr IMR-4064 load into small groups (usually 3 or 5 shots around an inch at 100 yards).

Cheers!
-Bob F.