FOUND IT! Here you go- from The Legacy of Lever Guns, Winchester 1886 by Dave Scovill. " The 38-56 can, for all practical purposes, be treated as a 38-55. The only difference is the 38-56 takes from two to three grains more powder to effectively duplicate velocitite and pressures produced by the 38-55. It's that simple. But, again, as with the 38-55, groove diameters in a 38-56 barrel can vary from .377 to .382." He later says his barrel was oversize at .381 and gave up on jacketed bullets.
Something to play with, his example of loads. For jacketed, back up a few grains and work up.
cast 274 gr RCBS 37-250
powder charge velocity
H4198 27.6 1547
XMR2495 33.4 1452
H4895 34.0 1671
XMR2015 29.4 1498
RL 15 34.5 1570
RL 7 24.0 1385
RL 7 26.0 1415
RL 7 30.0 1619
H4895 31.0 1377
220 gr Barnes RN .375"
RL 7 34.0 1750
Hope this helps. Use extreme caution. I've tried to rechk my tie pin and spel in but realize this is sent by some unknown idjot wit week eyes goin nuts on a funni typ riter.