There are 24 total guns in the game. Most of them can be found in your bonus menu after you complete the game, so if you miss a few as you go along, just pick them up from the bonuses. Note that the turret and the DShK cannot be selected from the bonus menu, so you'll need to pick those up in the campaign or through Chapter/Encounter select.
1. Para .45 - standard pistol first found in Chapter 3
2. INSAS - assault rifle first found in Chapter 3
3. FAL - assault rifle first found in Chapter 3
4. Micro 9mm - mini Uzi first found in Chapter 5
5. Mazur LDR - sniper rifle first found in Chapter 3
6. Silenced Pistol - generally found in lockboxes; first found in Chapter 3
7. Krivosk-XS - scoped pistol found in lockboxes; first found in Chapter 4
8. RPG - found on enemies and in lockboxes; first found in Chapter 4
9. Mettler M-30 - scoped rifle first found in Chapter 3
10. Arrowhead A3c - assault rifle found in lockboxes and on enemies starting in Chapter 4
11. Desert 5 - handgun found on enemies and in lockboxes starting in Chapter 4
12. Spezzotti 12 Gauge - shotgun first found on enemies in Chapter 4
13. P90 - assault rifle first found on enemies in Chapter 4
14. China Lake GL - grenade launcher found on enemies and in lockboxes starting in Chapter 4
15. Raffica - 3 burst pistol first found on enemies in Chapter 4
16. Type 95 - assault rifle first found on enemies in Chapter 4
17. S-12 Shotgun - first found on enemies in Chapter 5
18. Harbinger Sniper - first found on enemies in Chapter 7
19. Pistole - first found on enemies in Chapter 8
20. US-AN 12 Auto Shotgun - first found on enemies and in lockboxes in Chapter 8
21. RKL-155 - auto pistol first found on enemies in Chapter 8
22. Carrigan 53 - assault rifle automatically on you at the beginning of Chapter 9
23. DShK - minigun found on ultra heavy enemies; first found in Chapter 7 (Chapter 4 if you are on Crushing)
24. Turret - first found in Chapter 4; you'll use it as part of the main story in Chapter 9