Even if it fires, it wouldn't have much velocity. Large bullets have a lot of mass, so they need a lot of fuel to burn in order to get them up to speed. This fuel burns relatively slowly. That's why large guns usually have long barrels as well. That allows for the fuel to completely burn and the expanding gasses to keep pushing the bullet all the way. The barrel can be too long, so usually it's made such that a little bit of unburnt fuel comes out as the bullet exits. But with a small little thing like this, almost all the fuel would be thrown out with the bullet.

It would still be dangerous obviously, but I think much more dangerous for the person operating the device.

recoil right into your forehead