Carl, the answer depends on which version of 4.2"-caliber (30-pdr) Broun you're talking about. The 4.2" "Improved Broun" (Bell's page 199, found only in the Richmond VA area) has a perfectly flat base. The version shown on Bell's page 198, found only in the Mobile AL area, presumably made by Selma or one of its contractors has a somewhat rounded base... which more accurately would be called "low-convex."
Regards,
Pete