Dragoon

From: Assault rifles
Price: 400 Lei (Ethan, once unlocked)
Chapter: 12 · Chris: the village and the caverns · Ceremony Site

Power: 350
Rate of fire (s): 0.11
Reload (s): 2.9
Capacity: 30
Slots: 10 (5×2)
Ammo: Rifle Ammo
Upgrades: none, no parts
Infinite ammo: 50,000 CP (owning it is enough)

How to get it

Chris: he carries it from the start of ch12, with 3 boxes of 100 Rifle Ammo. Ethan: Extra Content Shop, 30,000 CP after you finish the story; then buy it from the Duke.

Verdict

With Chris, fire freely (1,500 rounds in the mission); on Ethan's second run, Dragoon + infinite ammo (80,000 CP) is the cheapest fully automatic gun with infinite ammo (the USM-AI with its own costs 60,000 CP).

Chris's resupply crates (next to the flares): Luiza's Yard (Rifle 2×100, Flash Grenade ×2); Workshop Yard (Rifle 3×100, Hand Grenade ×1, Med Injector ×1); Leonardo's Front Yard (Rifle 2×100, Hand Grenade ×1, Med Injector ×1); back of Eugen's Yard (Rifle 2×100, Hand Grenade ×1, Flash Grenade ×1, Med Injector ×1); Maiden of War crater after the strike (Rifle 3×100, Hand Grenade ×3, Med Injector ×2).

For Ethan, Rifle Ammo comes from Recipe: Rifle Ammo (10,000 Lei once you own the Dragoon or the WCX): 1 Rusted Scrap + 1 Metal Scrap + 1 Chem Fluid = 20.

Dakka Dakka challenge: 180 assault-rifle kills (27,000 CP).