Abraham Meleha has armed himself with a bolt action rifle, which means he has to manually operate the bolt to extract and reload a fresh round into the chamber. In his fire fight with McGarrett his rifle fires rapid fire like a semi automatic. The sound effects are in fact that of a M1 carbine semi automatic rifle.
During the climax, you can see a crew member in a white polo shirt appear in the hallway exit by the bomb truck, behind the actors portraying police officers.