No this would probably not be a workable idea. While the two are identical or extremely close in several dimensions, my copy of the Hornady loading manual, Fourth Edition, shows the case mouths and amount of taper differ a good bit. The .38-55 tapers from .419" at the head to .392" at the case opening, whereas the .375 goes from .420" to .400."
Besides what Bill mentioned I can add that I have been loading BTB 250gr LFNGC in 38-55 cases over 35.4gr of Re7 and shooting them in my Marlin 375. Whether these same loads would be safe in a new Marlin 1895 Cowboy (or whatever they're called) chambered for 38-55 I don't know. Assuming they'd chamber ok there is still the question (ie rumored) about whether Marlin used special heat treating for their Model 375 or not. If so then this load might not be safe in the newer rifle. I'm beginning to note what appears to be failure of the 38-55 length case to open completely now after about 6 loadings...time for a trim. Watch out for that. Also be careful of which length 38-55 cases used...there are at least 2 lengths that have been available over the years. Check out www.buffaloarms.com.
Disclaimer: The above load was safe in my rifle. Reduce by 5% for starters.
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