My very early Wesson & Harrington Target Model is .374"x.3775". I've heard of very few that measured so small though. My old rifle shoots pretty well with the RCBS gas check bullet, not so well with any plain base I've tried so far. I have three Lyman, one Mountain Molds, and one Lee custom mold in plain base, and that selection makes me think plain base is not the way to go with this particular barrel. I've also tried two brands of commercial PB cast bullets, and they were worse, both brands sized at .379".