This thing is the big gun of doom. The big bang.
For Reference, the historically most powerful weapon in the game until the latter packs of Forgotten age was the Lightning Gun, it still remains on the short list of most powerful weapons in the game.
Compared to Lightning Gun Chicago Typewriter deals the same amount of damage, has one extra ammo and costs one less XP and Resource. The only disadvantage is the drastically lower attack bonus of +2 instead of +5, this is a big issue because most characters start from 4, so the combined attack is +9, a will land all of its's Lightning bolts straight up with no help from anything else. The usually starts from 3 and the +2 puts them at +5, an unimpressive number to say the least, the extra action pump can remedy that partially. Other sources of raw boosts are required to reach proper accuracy.
Some simple upsides to Chicago Typewriter:
Deals 3 points of damage per shot, mediocre in some campaigns (Core, Dunwick) great elsewhere (Forgotten age). Abilities to bump that damage to 4 are important but they are somewhat hard to find, Vicious Blow and Beat Cop are options for some, others will cost you separate actions, for example Hatchet Man or Coup de Grâce.
Contraband is a natural combo, so is Extra Ammunition. "Skids" O'Toole can use Well Prepared to get a +2 boost from Chicago Typewriter's 2 icons. If the damage tick from Coup de Grâce isnt helpful then you can use it's 2 to land hits.
Jenny Barnes has the money to pay for it, she can emergency tech in some of those cards from to make the hits land. Jenny Barnes is probably the top character for utilizing the big gun right now. Until the next deluxe that is.
All in all, Chicago Typewriter is a great card, it's just in a weird spot where there aren't (good)characters around for whom it's a natural 1st priority upgrade. Thankfully this problem will be remedied in the Dreamlands deluxe where a 5 will be unleashed.