Poor accuracy past 50yds
I am shooting a 458x2 with a 340gr gas check boolit. The boolit is an Accuracy 46-335 AG. It is loaded with 33gr of 4759. At 50 yards 20 rounds will produce a 1.05 inch hole measured edge to edge. by my calculation it is a .59 in group at 50 yds. The problem is when I move the target out to 100yds the 5 shot groups open up to 3 to 4 inches. What can I do to improve the 100 yard accuracy? As I decrease the powder charge 50 yd accuracy suffers. I don't want to go to much heaver powder charge without published load data. I have been using 45-70 Win 1886 data from the lyman 4th edition Cast Bullet Handbook as a general guide line.
The rest of the information
gun - Mauser action
20" barrel 1-18 twist
Grove - .458
Bore .450
Throat .462
boolits sized .462
Alloy mixture Bhn 14-15
Lube FWFL
Help please
Carl