I have a T/C Encore in a multitude of calibers and am having extreme accuracy issues with my .450 marlin barrel. I was hoping someone could help. The barrel in question is 18 inches with a vais brake and was a rechamber from the 45-70 which is factory available. I am fairly new to handloading and have been able to tweak the loads to meet my expectations for all other barrels. I have tried 4 diffrent bullet weights and 3 diffrent powders and cannot get acceptable accuracy beyond 25 yards out of the 450. Any advice would help.

have the barrel inspected.... make shure who ever ran the 450 reamer into the chamber did it stright.....a crooked chamber could suck real bad

but over all id send it back and see what the issue is. if you cant get accuracy at 25 yards then its a barrel tolerace issue not a reloading issue.

If you're getting 1 inch groups at 25 yards, you should be getting no worse than 2 inch groups at 50 yards or 4 inch groups at 100 yards. If your 50 yard groups are large and out of proportion to what they should be like say 4 inches, then the problem is the load. It might be that the velocity isn't high enough to stabilize the bullet, the twist isn't adequate, etc. Some bullet types don't lend themselves well to long range shooting.

You didn't say whether or not your barrel is scoped. If it is scoped, make sure the mounts aren't loose and that there isn't something wrong with the scope. Heavy recoiling handguns are hard on scopes and it's entirely possible for the scope to be at fault.