Spend like 3-4 motes to level up the last sight/barrel node on blue and legendary special and heavy weapons and select the last node. Once you do that, then dismantle. You will get more parts, motes, and marks (if legendary) than you would if you just dismantled them off the bat. Primaries take like 5+ motes and are a waste since you probably won't get 5 motes back when you dismantle them. For specials and heavies, you'll get 1 to 3 motes, and 2-5 weapon parts per weapon (usually 2 to 4motes total, and 4 to 6 weapon parts total). I've gotten 5 weapon parts from new and old legendaries a few times, and for the new legendaries you can get up to 5 marks just for having the barrels or sights upgraded. Costs next to nothing other than a few motes (which you mostly get back) or a couple bounties. I usually stick a primary I want to keep and level up in the primary slot and throw a blue heavy and special in their slots, pop 3 motes, level up the sight/barrel column, select the last node, then swap them out for another blue special and heavy and keep doing it until I'm out of weapons to dismantle. Rocking court of Oryx for 30+ mins and/or running strikes will get you tons of armor and weapons to do this with. The best part is if you're doing it right, you get the motes back you spend on leveling up the 1st column most of the time, or it ends up costing you a mote if rng is being stingy.