Vampyr

All credit for this guide goes to SnowNinjaRaccoon (DK) & PowerPyx from PowerPyx.com who provide a complete guide to finding all collectibles and all weapons in Vampyr. This guide covers all 11 ranged weapons in chapter order, although the guide they created also covers all collectibles and melee weapons as well.

Prologue:
[Ranged Weapon #1] Used Revolver
Granted automatically in your shelter at the beginning of the game (upper floor).

Chapter 1:
[Ranged Weapon #2] Used Lupara
After the bar, the boat corpse and switching to other side of the docks. In a chest on the second floor of the second warehouse you will go through.

Chapter 2:
None

Chapter 3:
[Ranged Weapon #3] Common Pistol Parabellum
Granted for defeating Jimmy “The Spark” boss fight. (Flamethrower guy in the docks, forced fight).

Chapter 4:
[Ranged Weapon #4] Remarkable Pump Action Shotgun
While investigating the source of epidemy in West End, you will find it in the eastern house you will have to visit.

Chapter 5:
[Ranged Weapon #5] Good double-barrelled
2nd floor of the shopping mall in the north of West-End district. You must defeat the enemies here and then teleport upstairs to the chocolate store on 2nd floor (you will go through the shopping mall during the story on your way to the park).

Chapter 6: Important: Make a manual Save Game Backup at the start of Chapter 6! See spoiler:
[Ranged Weapon #6] Milton’s Shotgun
Embrace Milton at the Pembroke Hospital (alternatively, it can also be bought from him).

[Ranged Weapon #7] Clarence’s Parabellum
Embrace Clarence in West End.

[Ranged Weapon #8] Joe’s Semi-automatic Pistol
Embrace Joe’s son Harry Peterson in Whitechapel.

[Ranged Weapon #9] Albert’s Revolver
Embrace Albert Palmer in Whitechapel.

[Ranged Weapon #10] Edwinas Double-Barrelled
Embrace Edwina in the Docks.

[Ranged Weapon #11] Tom’s Revolver
Embrace Tom in the Docks.

For weapon locations, I refer you to TheDustyCactus's solution. But I had a few unanswered questions that I had to find out for myself, so in case anyone else is wondering:
Yes, you must get all the guns in the same playthrough. If you mess up like I did and a district goes hostile before you bit the last citizen who holds a weapon you need, you're SOL. Simply getting that last weapon on a second playthrough won't be sufficient - you need to get ALL the guns again in the new playthrough.

The good double-barreled shotgun can be acquired as early as Chapter 4. As soon as you have access to the West End, you have access to the area where you can pick up that gun. However, you will unfortunately still need to play through the first section of Chapter 5 before you can unlock this achievement, because Edwina is mesmerize level 5 and you need to deal with Dawson in the mission "Unnatural Selection" before you get to that level yourself.

Yes, this CAN be earned while playing in "story mode." Again, in case you missed this on your first run and need to start a fresh one, playing in story mode makes it a bit simpler if you want to speedrun through and just collect weapons. There is still combat, but it's baby mode.

Welcome, Here is videos walkthrough for all weapons and collectibles, Enjoy

Credits for youtube video goes to PowerPyx channel