Are you sure the difference isn't caused by how your bullet fits into the chamber? A different bullet, one that has a bore riding portion that sits on the top of the lands may well seat deeper into the chamber and allow a longer tube/more powder. An easy way to check is to try to insert the bullet into the muzzle. If it fits to the top driving band and shows no "wiggle" aka looseness where it sits on the lands, that's a good fit. If it won't go in to the top band, that's why you can't use your longer tube.
If it was me, I'd slug the bore and see what I've got.
PS - I went through this last year with a Garrett I bought from a reenactor. I'm using a Moose 544-480 Ringtail mould, a Hahn 1 3/8" tube and 40 grains of Goex 3F. I had Larry Flees do a chamber sleeve and gas check job and he profiled my chamber to fit my bullet. I use the exact same load in an old Shiloh Sharps with the same Dobbin gas check and sleeve.
Bob Anderson
Ordnance Sergeant
Company C, 1st Michigan Volunteer Infantry
Small Arms Committee
"I won't be wronged. I won't be insulted. I won't be laid a hand on.
I don't do these things to other people, and I require the same from them."
- John Wayne in "The Shootist", 1976
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