There are 12 different guns in the base game that you have to infuse. Add-on weapons and the machete do not count for this trophy:
The guns are:
1. PT-6 "Spiker"
2. Tribunal
3. LIMP-10: this is the SMG you find in the starting area and is thus available at the beginning of each loop.
4. The Fourpounder
5. Rapier
6. MG-1 "Pepper Mill"
7. Vopat Trencher
8. Strelak 50-50
These 8 guns can be commonly found throughout the game, carried by Eternalists or in the environment.
The last 4 are gold-tier, and are most reliably found by completing their associated Arsenal Lead quests.
9. Constancy Automatic: You can loot this from Frank's body at Fristad Rock in the morning.
10. Strelak Verso: When entering the Complex at noon, you will hear a series of explosions. You will find that Eternalists have blown open the Ordnance Depot. Continuing forward into the Depot, you will find a locked door that needs 4 batteries to be opened- and the Strelak Verso on the other side. Find 4 batteries to open the door and the dual guns are yours.
11. Sepulchra Breteira: While exploring Updaam in the afternoon, an Angry Eternalist will get on the announcements and tell you that there is a wrapped gift in front of your apartment in Dorsey Square. If you open the gift, Eternalists will start shooting at you from nearby buildings and the statue. The sniper rifle, Sepeulchra Breteira can be looted from an Eternalist found in the apartment above the bakery.
12. Heritage Gun: If you enter Karl's Bay at night, you might hear two nearby Eternalists examine a ticking box that soon explodes. A message from Charlie Montague on the announcements will tell you about his game, Reward Scheme. To find the locations of the other boxes, head to the Fathoms of Lament building. After killing the Eternalists there, you will find the Heritage Gun in a locked case and a map with blinking lights. You have to find the box at each of the blinking lights and open them without destroying them (careful, some might explode upon opening!) The entire challenge is not timed, but some of the boxes are when you start their puzzles.
This guide here has more information:
https://www.ign.com/wikis/deathloop/Heritage_Gun_Quest_-_Sup...After opening all the boxes, a door at the back of Fathoms of Lament will open, and you must complete one final puzzle by hitting switches and avoiding lasers before the time runs out. When this puzzle is over, the Heritage Gun will be unlocked for you to grab.
All guns may additionally be found by killing and looting an invading Julianna.
Return to the tunnels after collecting the weapons you need to infuse, and make sure you have enough Residuum. The gold tier weapons each require 9,000 Residuum to infuse.