Resident Evil Village

Found a guide on the weapons that you get during your first playthrough and where to get their parts in case you need visuals so I am linking it below:
https://www.rockpapershotgun.com/resident-evil-village-weapo...

These are a total of 22 parts that count towards the achievement:

Lemi (starting gun):
Recoil Compensator (Hallway right outside the Kitchen in Castle Dimitrescu)
High Capacity Mag (9,000 Lei from the Duke)

M1897 (lying on a table when you're first attacked by Lycans in the Village)
Hair Trigger (8,000 Lei from the Duke)

F2 Rifle (found in the attic of Castle Dimitrescu, before you exit into the rooftop and fight flying enemies)
High Magnification Scope (15,000 Lei from the Duke)
Cheek Rest (20,000 Lei from the Duke)
High Capacity Mag (found in the village. Look for the Maestro's Collection treasure on the map)

M1911 (find it in a dial-locked compartment in the workshop in the Village. Code should be 07 04 08)
High Capacity Mag (found at Moreau's waterfront. Hard to miss since its in a case and on the way to an objective)
Improved Grip (12,000 Lei from the Duke)

W870 TAC (found in a house on the way back from House Beneviento)
Foregrip (Need the crank to get this part. lower the drawbridge by Lone Road on the map and drive the boat to a cave with the part inside)
Improved Gunstock (22,000 Lei from the Duke)

M1851 Wolfsbane (When first heading towards Moreau's Waterfront, there is a door that requires the crank, right next to the first windmill. When you acquire the crank, use it on that door and it will be down the path in the house on the right)
Long Barrel (found in the Operating Room right off of Heisenberg's Quarters)
Increased Capacity Cylinder (100,000 Lei from the Duke)

V61 Custom (120,000 Lei from the Duke. Late game weapon)
Long Barrel (56,000 Lei from the Duke)
Gunstock (35,000 Lei from the Duke)
Drum Magazine (70,000 Lei from the Duke)

SYG 12 (180,000 Lei from the Duke. Late game weapon)
Red Dot Sight (42,000 Lei from the Duke)
Long Barrel (64,000 Lei from the Duke)
Drum Magazine (88,000 Lei from the Duke)

The rest of the guns and their attachments are gotten during new game+

WCX (Purchased from the extra content shop for 30,000cp. Will cost 200 Lei to purchase from the Duke)
Foregrip (80,000 Lei from the Duke)
Red Dot Sight (140,000 Lei from the Duke)

S.T.A.K.E. (300,000 Lei from the Duke. Will appear in the Dukes inventory after you have completed the game once)
Improved Grip (120,000 Lei from the Duke)
High Capacity Mag (180,000 Lei from the Duke)

As a side note, you can save before buying a gun or component from the Duke and then reload the save after you buy and equip all the upgrades to the gun to keep your money but save the progress on the achievement. The game tracks it in game as you complete the upgrades but does not require you to keep them. This also means that you can buy individual parts.

You do not need to have all parts on a gun for it to count. You can check you progress if you go to Challenges>Veteran Gunsmith from the pause menu. The game won't tell you which parts you have gotten, so please keep track of them. This is how I saved myself a lot of hours of farming money. Hope this helps.

Not my video but this guy made an amazing collectable guide that lists every single file, outhouse, window, weapon and goat in the game its incredibly easy to follow and goes in order they appear so you can do a section pause and not be spoiled while getting 100%