Any .30 mold will work. Consult the .30-30 section in any data bank or manual, it's so close to the same as .303 that the data is interchangeable. Do a chamber cast to determine throat diameter if you can and size accordingly. Note: .303's were always .308 groove diameter from day one, they made their proprietary jacketed bullet ammo with .311 bullets for a couple short years at first to try to gain extra pressure/velocity but that was given up on as a dumb idea.

I've loaded cast bullets in mine from 110 grains up to 210. I have settled on a 190 grain flat nosed bullet out of an old custom Saeco mould, cast from clip-on wheel weights, unquenched, bhn 12 or so, sized to .310" (as dropped from mold), over 28 grains 3031. No pressure signs in any of my guns, but it's getting up there so start lower and work up if you feel the need to. No leading, ever, accuracy is superb, and deer die with alacrity. This one took three steps and keeled over, range 110 yards.

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For target work I use a 180 grain Walt Melander-era NEI round nose, same alloy and size, over 15 grains SR-4759. (Moot point because 4759 is discontinued, but I have enough stashed to last me to the end.) 5744, RL-7, 4227, 2400, etc. would be good choices also. Check your books.

My favorite plinking load is a bullet out of an ancient Lyman/Ideal mold- 311241. 155grain plain base cast from WW's and soft lead, bhn 8-9, sized .310 and driven by 5.5 grains Bullseye. I shoot probably 20 of these for every hunting load I fire. More fun than a room full of hookers. (Well almost anyway!)

"You can lead a man to logic, but you cannot make him think." Joe Harz
"Always certain, often right." Keith McCafferty