If Megagun is a mafia pretending to be Justice, his strategy is very risky; however, the fact that both the Gunsmith/Fabricator and Megagun visited Hooligan on the same night seems quite unlikely to me.
If Hooligan uses the gun and the gun is fake, Hooligan will die but we'll get info on someone (I'm not sure about this part). We will also know that guns can't be trusted.
If Hooligan shoots Jace and the gun is real:
If Jace is mafia -> good;
If Jace is town -> either Hooligan, Megagun, or both are mafia -> possible good for tomorrow.
If Hooligan shoots Megagun and the gun is real:
If Megagun is mafia -> good;
If Megagun is town -> either Hooligan, Jace or both are mafia -> possible good for tomorrow.
Imo Hooligan should shoot, but there isn't much time left.
As for the lynch, Idk.