The following list is of ALL the firearms that can be unlocked in the game and how to unlock them.

.38 Double Action
Level: 5
How To Unlock: Reach Loyalty Level 3 with Frederick Abberline.

Derringer Pistol
Level: 3
How To Unlock: Complete Sequence 3.

Pocket Pistol
Level: 3
How To Unlock: Purchase for £3000.

Model 1 Revolver
Level: 3
How To Unlock: Complete Sequence 4.

(54 Bore) 1856 Revolver
Level: 4
How To Unlock: Reach Loyalty Level 1 with Frederick Abberline.

Single Action Army
Level: 4
How To Unlock: Purchase for £4000.

Lancaster 4-Barrels
Level: 4
How To Unlock: Complete Sequence 6, 'A Case of Identity'.

Bullseye Revolver
Level: 4
How To Unlock: Uplay reward.

Demonic Revolver
Level: 4
How To Unlock: Redeem a Promo Code.

Stealth Revolver
Level: 4
How To Unlock: Redeem a Promo Code.

M1877 'Lightning'
Level: 6
How To Unlock: Complete Sequence 7, Unbreaking the Bank

M1877 'Thunderer'
Level: 7
How To Unlock: Conquer the Borough of City of London.

Model 3 Revolver
Level: 7
How To Unlock: Reach Loyalty Level 4 with Frederick Abberline.

Self-Loading Pistol Model 1868
Level: 9
How To Unlock: Reach Loyalty Level 5 with Frederick Abberline.

The Mars
Level: 10
How To Unlock: Reach Loyalty Level 5 with Ned Wynert for the Crafting Plan, £10000, 150 Metal, and 100 Chemical