Most guides will only go over how to actually do the achievement, but not how to build your weapons for it. Hopefully these builds will serve you well and make earning the achievement less of a pain.

Credits to KarateF22 and his wonderful Compilation of Optimal Weapon Loadouts guide, which inspired this one. Check it out here:
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=251751235

Some of the builds here require DLC packs, mainly the Gage Mod Courier and The Butcher's AK/CAR Mod Pack. If you lack one or both of these DLC packs, there are still alternative builds included.
The Fast and Furious bonus from the Perk Decks is included with the damage.

Butcher Pack + Gage Mod Courier Build
52.5 Damage, 18 Accuracy, 25 Stability, 24 Threat, 60 Magazine

• Modern Barrel
  
• Competitor's Compensator
  
• Scope Mount/Nothing
  
• Keymod Rail
  
• LED Combo
  
• AK Wood Grip
  
• AK Quadstacked Mag
  
• Any sight
  
• War-Torn Stock

Boasts maximum accuracy and stability along with a huge magazine size. The extra damage is insignificant, since it already hits the 40 breakpoint.

Gage Mod Courier Build
47.25 Damage, 18 Accuracy, 25 Stability, 24 Threat, 60 Magazine

• Competitor's Compensator
  
• Lightweight Rail
  
• LED Combo
  
• AK Quadstacked Mag
  
• Any sight
  
• Wooden Sniper Stock

Designed for people with only the Gage Mod Courier pack. This is virtually identical to the above build, save for less damage which doesn't even matter.

Butcher Pack Build
57.75 Damage, 18 Accuracy, 25 Stability, 34 Threat, 34 Magazine

• Modern Barrel
  
• Fire Breather Nozzle
  
• Scope Mount
  
• Keymod Rail
  
• LED Combo
  
• Aluminum Grip
  
• Low Drag Magazine
  
• Any sight
  
• Classic Stock

For people who lack the Gage Mod Courier pack. The higher damage and threat might make it seem better than the other builds, but once again it doesn't matter much. The magazine capacity also isn't nearly as high.

DLC-less Build
44.62 Damage, 18 Accuracy, 23 Stability, 24 Threat, 30 Magazine

• Stubby Compensator
  
• The Tactical Russian Handguard
  
• LED Combo
  
• Any sight
  
• Wooden Sniper Stock

For people who own absolutely no DLC. Other than the lower magazine size and stability, it should perform just as well as the builds above.

DMR Build (Mod Courier + Butcher Pack)
94.50 Damage, 18 Accuracy, 24 Stability, 24 Threat, 60 Magazine

• DMR Kit
  
• Competitor's Compensator
  
• Scope Mount
  
• Lightweight Rail
  
• LED Combo
  
• AK Wood Grip
  
• AK Quadstacked Mag
  
• Any sight
  
• Classic Stock

The extremely high damage will allow you to one shot Tans, at the cost of less total ammo along with a halved ammo pick up rate. You should generally be using this as a single shot weapon, though you can switch it to full auto if you need to take down a Bulldozer quickly.

Due to the Chicago Typewriter's limited unique mod pool, it relies on Barrel Extensions to change its stats in any meaningful way, which limits how you can build it. Only the Gage Mod Courier pack is necessary for some of these builds.

Balanced Build
28.87 Damage, 16 Accuracy, 25 Stability, 13 Threat

• Long Barrel
  
• Competitor's Compensator
  
• LED Combo
  
• Any sight

Takes down tan cops in 3 shots, and has good accuracy along with maxed stability.

DLC-less Build
28.87 Damage, 16 Accuracy, 21 Stability, 12 Threat

• Long Barrel
  
• Ported Compensator
  
• LED Combo
  
• Any sight

For non DLC-owners, this is identical to the above build except for the loss of stability.

High Damage Build
34.13 Damage, 10 Accuracy, 23 Stability, 24 Threat

• Long Barrel
  
• Funnel of Fun Nozzle
  
• LED Combo
  
• Any sight

For those who absolutely demand to one shot green cops. The accuracy suffers as a result, so this is generally suited for engagements at close range (particularly on Day 2 of the heist).

Once you've built your weapons, you'll probably want another guide to run you through the heist. Here's some examples:
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=359248748
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=258289639
If you have any suggestions or corrections for these builds, feel free to comment below. Thanks for reading!