The achievo requirement here is to “Master any one weapon”
So, for any rifle, pistol, machine gun or shotgun, you have to unlock all of the 12 upgrades, and also complete the mastery task. The mastery task for each weapon will NOT reveal itself to you until AFTER you have upgraded it 12 times....but I have written them for you below, so you can see which ones you might prefer and which you might not fancy doing at all.
NOTE: There is also another similar achievo that you get when you have mastered 3 weapons : (it says “ALL”, but it means “3” :
The 12 upgrades are quite straight forward: play the campaign and collect upgrade crates. Some upgrade crates are gained by levelling up too.
The mastery tasks are more tricky, and are all variations on “kill a certain number of enemies in a certain manner”. They are made easier by the fact that you can restart a checkpoint, or restart a safe room, and you will keep your kill tally, so they can all be farmed relatively simply.
Here are the mastery requirements for the 12 weapons in the base game:
Gewehr - kill 10 snipers through their scope
M1 garand - kill 100 heavies, officers or blind screamers by shooting their weak spots
Mosin nagant M91/30 - get 200 headshot kills from over 50m
M30 drilling - kill 250 zombies by dismembering limbs
MP44 - get a kill every 2 seconds for a total of 30 kills
Thompson - shoot and dismember the legs of 200 zombies
Trench Gun - get 200 triple kills or above
C96 pistol - get 200 ricochet kills
M1911 - get 200 headshot kills
Webley MKVI - get 200 double kills or above
Here is where I farmed some of the mastery kills:
Gewehr rifle. DEATH CANAL “Carnivorous Canal Cruise” where you first meet snipers in the game, at the point where the boat stops and you have to refuel the boat. Easy difficulty. Be patient. By the time you have done a few, it will seem much easier: Kill a sniper through their scope, and RESTART checkpoint. Repeat as necessary.
Trench-gun, webley pistol, MP44 machine gun all farmed at the same place: DEAD AHEAD ”Locomotive Lockdown”. Go through the safe house gate, climb up, go through the next gate, skip the cutscene, and, whilst on the sloping gangway, try to get some multikills until almost all the zombies are gone (before the next checkpoint ), and reload checkpoint. Easy difficulty with enemy number set at “4 players”.
Hell Base “Steal the Relic” has also been suggested (by iRogueMilitia ) as a good farming chapter, as enemies will steadily stream towards you through a set of double doors.
Achievement will pop as soon as your weapon has had it’s skill mastery task completed.