Fill your inventory with all green weapons from the gunsmith and play the level 18 strikes. You can level up about 2 guns per category each strike, which means about 6 per strike. Once they are leveled up, they dismantle for at least 2 parts, but I've seen some dismantle for 4. So if we do the math, you can basically get 12 weapon parts per strike, more if you have the nightfall buff and used a telemetry, plus any engrams that drop. Easiest to do with green weapons, as blues take FOREVER to level up and don't provide much more parts. This can be done with vanguard helmets as well for armor parts.