I would think a short slug like the short English slugs of about .641" would weigh at or just under 500gr. 1 1/4 ounce at 545gr. would probably be a maximum. The heavier the slug, the higher the pressure but I suspect even at 1 1/4 ounces the pressure would be under 22,000psi.
A .615" to .620" ball in thick denim would make a clean shooting load - middle row.