I have a Marlin Cowboy 38-55 with a 1/18 twist. I have been shooting 250 & 260gr cast bullets quite accurately. Was wondering if the 1/18 twist would stabilize a 300gr bullet ? I would prefer to shoot the heavier bullet if I could do so, accurately.

Well answered my own question. Found a .380 dia, 290gr bullet mold. Bullet length, 1.07, vel 1400fps . Out of my Marlins 1/19 twist bbl. Stability rating of 2.300. Far more than needed. https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi