It would cause the rapid emission of various kinds of radiation. The thin amount of lead pictured would not meaningfully shield the user from the more penetrative radiation, in particular they would get hit by a looooot of fast neutrons. Some of which they would have the misfortune of slowing down as water is an excellent neutron moderator. These slow neutrons would then kill the person before they knew what happened.

About 2 weeks later their body would catch up to their status as a dead man walking and it would become official.

At the other end of the device the target would suffer fates ranging from nothing, mild increases in rates of cancer, all the way to similar status.

It would not be very useful as the beam isn't particularly collimated and so would lose effectiveness over distance rapidly. As well as the obvious downside of requiring a suicidal user.

the inside of the barrel should be made out of beryllium or another neutron rrflective material for more damage but otherwise seems good.