That gun is close to the Jabba the Hutt blaster that Luke tries to shoot Jabba with. The one thing that I know is wrong is the front site is different. The rest of it looks close. It may be a similar model or just differently configured. Hopefully someone who knows it better will chime in.
My friend had one like that back in 1984. It had the correct end on the barrel. The bottom part of the front of the barrel was a lever to pump the gun up to shoot, it was hinged at the base of the barrel.