I'll try to help. The .38-55 Is in my opinion, one of the sweetest rounds out there. I took a Large Doe at 150 yds. just a few week ago with it.
There "IS" a lot of frustration going on with some shooters with real .38-55 chambers and bores.
This is because some ammo manufactures for some reason, can't or wont make it right.
Winchester for one, and its loadings are using .375 diameter bullets.
I would first slug the barrel. This is just to find out what you are actually dealing with. (Probably somewhere around .377-.379..)
You can use .375 win brass But, You will need to turn the necks as they are too thick.
Starline makes real .38-55 brass at 2.125" that when loaded with a cast .380-.381 slug will still chamber.
HTH.
For Factory loaded ammo, I found Ten-X 260 gr, to be great stuff.
They are using .379-.380 Dia.