Instructions
Level your character into the high 40's. Sometimes the most powerful weapons may not be rewarded until reaching the level cap at 50. Recovered weapons get better as the character levels and the best SMGs are usually recovered in the high 40 levels.
Recover weapons whenever possible. Since weapons are randomly generated, the more you obtain, the better the chances of finding a strong SMG.
Look for Double Anarchy SMG weapons. Double Anarchy are the most powerful of the conventional SMG models in the game, with only the 4X Combustion Hellfire having more power. While the weapons lack range and accuracy, they fire four bullets at once for high raw damage. This model of weapon begins to appear before level 10, but the most powerful are found at higher levels.
Check for a good rarity of weapon. The rarer the weapon, the stronger it is. Purple quality items are very strong while orange are stronger and pearlescent are the most rare and therefore strongest. Rarity also means these weapons are most likely to take the longest to acquire.