I'm sure you've read the 38-55 info over at marlinowners already but with a .380 bore, I think you'll be limited to cast. Rimrock bullets has a good selection of bigger cast diameters. Unique will often provide great accuracy and good velocity with those cast bullets. Working up to 10 grains or so seems to be the magic formula. Tremendous amount of info regarding 38-55 in these threads and sticky's:
http://www.marlinowners.com/forum/32-40-38-55-375-other-great-non-standard-calibers/
