3Trespassing with firearms in a building

Any person who, while he has a firearm with him, enters or is in any building or part of a building as a trespasser and without reasonable excuse, the proof whereof shall lie on him, shall—

(a)

be liable on summary conviction to imprisonment for a term not exceeding six months or a fine not exceeding two hundred pounds or both;

(b)

unless the firearm is an air weapon, be liable on conviction on indictment to imprisonment for a term not exceeding five years or a fine or both.