Magnum Ammo

Only 14 rounds placed in the whole game:

• Luiza's Yard ×2: barrel by the gate, after you return from the castle.
• Control Platform (Reservoir) ×1: breakable crate by the white switch.
• Trap Chamber (River) ×1: under the torture bench.
• Riverbank Treasure House, Chamber C ×1.
• Stronghold Tower 2F ×2: wooden boxes on the west side.
• Private Passageway (Factory MB4) ×3: locked drawer (Lockpick).
• Large Power Regulator Room (Factory MB4) ×2: wake the Soldat Eins on the table so it breaks the fence.
• Altar ×2: SE wall, after the Factory.

Village of Shadows adds 2 at Moreau's Laboratory: on a board over two barrels, on the north side of the East Building.

Ammo for the M1851 Wolfsbane and the S.T.A.K.E.

Gunpowder ×4 + Rusted Scrap ×4 + Metal Scrap

There is no recipe on a first run: it appears in NG+ (10,000 Lei, makes 3) after you buy a magnum. Game8 and the wiki list 10,000 Lei a round at the Duke's shop (which playthrough is unconfirmed). 10 per slot: save it for bosses.