No reason that I see that prevents you from shooting your 1 inch mortar inthe Golf ball mortar contest...walk the walk!!!
DD - Forgive me since it's really none of my biz (As I won't be competing in "Walk the Walk"), but shooting lead or steel balls gives (In my humble and unsolicited opinion) an unfair advantage over shooting golf balls. Golf balls tend to bounce, non-bouncy lead or steel tends to stay planted where it lands.
You might consider creating a different Walk the Walk category for "OTGB" (Other Than Golf Ball).
I realize it's all in fun, but I've seen some folks get all bent out of shape (Justifiably at times) at Pinewood Derby races where officials made the slightest changes to the rules after cars had been made or racing had begun.
I'll go away now...