All things being equal a cast bullet should be .002 bigger than the bore. Have a way over bored rossi 92 44 mag made in 1993-4 that has a .434-.435 inch bore depending on how one measures the slug. It key holed at 25 yards. Switched to softer Desparado Bullets from cowboybullets.com 240 grain .430 diameter the rifle now shoots round holes at 100 yards and a 2.5-3 inch group from a 20 inch carbine iron sights and over 65 yo eyes.
thought you might want to know, who ever installed the barrel needed to put it at least one threat tighter has something like .030 too much head space, a buddy is going to fix it.
Some decent powders for .38 special are bullseye, red dot, A2, Nitro 100 on the slower side HP38, unique or universal. I use the Bullseye, Red Dot, A2 and Nitro 100 range of powders and get 2000 or so rounds out of a lb. not necessarily container as many now have 12 or 14 oz vs a true pound