The Thompson is a machine gun unlocked for purchase in the Chicago gun shop after completion of Act 1: "Bloody Newcomer". It is extremely powerful, being able to take out any NORMAL enemy (not bosses) with one shot at full upgrade level. Please note: This gun is unlocked for PURCHASE, you do not get it for free and still have to buy it.
Total Ammo: 150
The odd thing about this gun is that it never runs out of ammo in reserve, no matter how many times you reload. It is unknown whether this is a glitch or intended by the creator.
By completing Act 1, then starting another act on the same account and buying the gun in the Chicago Gun Shop. You Can get it by Typing in a cheat treasure to unlock the basement and go to the guns shop to buy it