Special Effect(s):Sets ammo pickup to 2.25-4.13 (AK), 0.45-1.58 (AK.762), is not affected by skills/perks
In the Firestarter job, complete the heist on OVERKILL difficulty or above, with all days done in loud and with a crew of 4 players using unmodified "AK Rifle" rifles and "PARA" submachine guns. Unlocks the "DMR Kit AK.762" and the "Low Drag Magazine" for the AK weapon family, "Timothy" mask, "Prehistorical" material and "Dinosaur Stripes" pattern.
The barrel appears to be based off the barrel of the Zastava M76, as evidenced by the string IDs referencing the textures for it bearing 'zastava' and 'm76' in their names.
Operation-wise, it appears that this mod was based on the Zastava M91's barrel instead, as the 7.62x54mmR cartridges it fires are of the same diameter as the 7.62x39mm rounds used by various AK derivatives which in turn enables them to use the barrel mod.
The barrel alone does not usually indicate what designation the weapon has, the DMR qualification usually encompasses the rifle's various accessories such as bipods, grips, scopes and stocks in addition to barrels. A more accurate name for the in-game mod would be DMR Barrel.
The Here Comes the Pain Train achievement refers to the modification as the "DMR Kit AK.762", suggesting that it was originally only intended to be equipped on the AK.762 and not the AK.