Bottomline up front, my assumptions on bore size of .423/.424 were erroneous. I have been resizing to far down.
Based on some good input on NitroExpress.com I reslugged the bores from both breech and muzzle. I sent the slugs and fired cases to a more skilled and better equipped person who could measure them and provide a good check to me driving myself crazy with inconsistent measurements.
The slugs measured out indicated the bore is right about .428" tapering about .001" toward the muzzle. (so the 0.423 was too small)
The fired cases are .434-.435" inside the neck, so the chamber has good neck clearance to release the bullets.
I test loaded two cartridges before I left the house this morning with bullets resized to 0.427 and they both came off correct without loose bullet tension. I did note that advice on the expanding button might be oversized and that maybe true
I intend to shoot it with the jacketed 0.427 bullets to see how they do before I move up to anything larger diameter. I am not going up to 0.428 not because I lack faith in the measurements but because I want to take it step by step.
I need to cancel my order for 0.423 258 bullets from Hawks as they will not be of much use.
Once I have nailed down the best regulating load with the Hornaday XTP 240 grain, I may go ahead and see how it does with the Hornaday FTX 265 Grain at 1800 FPS
It will be two weeks before I will be able to get to a range again to fire my next lot of reloads
