I haven't seen any .30 mm.
But there's a good chance he was shooting .030-inch natural latex. That's still available from Simple-Shot. That converts to a little over .75mm.
But, heck, if you're shooting 3/8-inch steel, I suspect any light latex tapered 3/4-inch to 3/8-inch, or even 5/8-inch to 3/8-inch would provide plenty of power for a short draw at close range. You could go narrower with lighter ammo, of course.
But there's a good chance he was shooting .030-inch natural latex. That's still available from Simple-Shot. That converts to a little over .75mm.
But, heck, if you're shooting 3/8-inch steel, I suspect any light latex tapered 3/4-inch to 3/8-inch, or even 5/8-inch to 3/8-inch would provide plenty of power for a short draw at close range. You could go narrower with lighter ammo, of course.