Dave,
An interesting picture. Is the base of the bullet cupped? I am thinking not as the .377" diameter in the .378" barrel allows enough gas to slip by the bullet to prevent this.
With the .377" diameter bullet I would consider the 33.0 grain load maximum. If you switch to a jacketed bullet of larger diameter I would lower the powder charge.
When working with the .30-30AI we went to .422" on many ocassions with no indications of trouble - but - This was clearly a greater pressure indication than we felt comfortable with for hunting loads at all temperatures. There is no point in looking for a problem with your hunting loads just for a few fps increase.
Looking at my load notes for cast bulets I found some stuff on a John Taffin article on the Marlin .38-55 Cowboy rifle from the Feb. 2000 issue of Guns magazine. John used a load of 30.0 grains of Reloder 7 behind the 250 grain RCBS gas check bullet and got 1758 fps from the 24" barrel of his Marlin Cowboy rifle. He sized his bullets to .379". John tried bullets of .377 and .379" and said he could not see a difference in accuracy - I assume he was using only the factory sights.
While it has been awhile since I have been there, I believe John Taffin has a web page linked with Sixgunner.com and many of his articles are posted there.
My notes indicate John called Marlin and Marlin says their .38-55 rifles have a groove diameter of .378".
