I finally made three careful slugs of the bore of my micro-groove Marlin 1895 in .45-70: under the front sight; under the rear sight; and full length.
The diameters as measured using a Mitutoyo micrometer showed a slightly oval bore with measurements ranging from .4589 to .4591. Not a huge discrepancy, but I think it shows why .459 cast bullets were horribly inaccurate. 405-gr cast, gas-checked bullets sized to .460 from Beartooth Bullets showed improved accuracy. Jacketed 300-gr bullets showed a reasonable accuracy.
Assuming I know how to use a micrometer, and assuming my micrometer is calibrated correctly, what are the chances that accuracy can be improved from the current 3-4 inches at 100 yards (scoped and from a rest)? For instance, getting a mold and sizing bullets to .461? (I called Beartooth, but they cannot supply .461 diameter cast.)
My suspicion is that it would be extremely expensive to rebarrel the rifle. Unfortunate since Marlin finally saw the light and changed to Ballard rifling.
Thank you,
Richard