El Tio Loco,
Yes indeed, a 3" group at 100 yards is very good...for a vintage Winchester with less than a perfect bore shooting b.p.
My original '73 Winchester (1882 mfg) has an oversized barrel as well (.435" avg). With 427098 bullets at .436" diameter, using Swiss 2F / CCI 300 primers / SPG, it will shoot about 16 rounds before accuracy starts to deteriorate.
Perhaps the fact that your bullets are made from w.w. and are .003" under groove diameter, may cause the earlier loss of accuracy.
For me, at least, w.w. are a bit too hard to bump up adequately. I would suggest mixing w.w. with lead (if you have some) 50/50 or trying some bullets from pure lead, but you would have to pre compress the powder before seating the pure lead bullets. You could use your neck expander for that.
The other option is to acquire a mold that would make bullets of at least .431" (.432" would be better) if the chanber would accept a cartridge with a bullet of that diameter. I am lucky in that my '73 will accept a cartridge with a bullet large enough to fill the groove.
I did try the 43-215C bullet which holds additional lube in my '73 and that extended the accurate shooting to 30+ rounds in the rough barrel with Swiss powder.
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