CREATE TABLE table_35344 ("Designation" TEXT, "Bore" TEXT, "Weight" TEXT, "Service Charge" TEXT, "Range  ( yards ) " TEXT, "Number Made" FLOAT)

What is the bore of the boat howitzers with a 12-pdr heavy designation?
SELECT "Bore" FROM table_35344 WHERE "Designation" = '12-pdr heavy'
SELECT "Bore" FROM "table_35344" WHERE "Designation" = '12-pdr heavy'