You should slug the bore of that milsurp first thing.
Barring that, I would get some cast bullets about.314 diameter and load 3 or 4 grains of Bullseye,700x or Red Dot or a fast similar powder and shoot at 20 to 25 yard targets and practice to shoot the smallest group.
This will give you a feel for how the gun shoots cast bullets and give you more confidence in the gun.
Stay away from 100 yard shooting for now.
These slow bullets taking their time to wander down the bore take quite a steadg hold for100 yard shooting.
Learning to hold steady at close range and shoot them all thru the same hole or a one inch hole can reach you to hold steady for those 100 yard shots.
A two inch group at 100 yards rather than an 8 inch group all over the target.
Lead .32 pistol bullets work real well for .303 Brits.