The 38-55 is a great cartridge but being chambered for over 125 years the guns that shoot it vary a lot.
You really need to start by slugging your bore first as if you have a true 38 cal. bore you will be wasting your time trying undersized bullets of .375 to .377 dia.
Once you know what size your bore is ( groove dia. ) you can start with regular starting load data that is easy to find online. It can be loaded very light with fast burning shotgun or pistol powders and cast bullets up to rifle powders in the burn range of 4198, R 7, 2015, 3031 ect.
There are two lengths of brass for the 38-55, most rifles are chambered for the longer 2.1"+ brass but will work fine with the shorter 2.080 or even blown out and resized 30-30 brass. There's a lot of possible combinations with that cartridge from plink ears to full big game hunting loads.
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