More like 5 or 6 parts per gun.
I order as much as I can with my 3 characters, on arm day I level up the weapon with bounties and Motes of Light.
You get 5 marks, 2 motes and about 5 parts for 2500 glimmer and a few motes
English
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Using the bounties they will level up eventually. For this week I could get a primary, secondary and heavy weapons equipped and used between 5 and 15 motes
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Yeah for upgraded ones, I'd rather not waste 2500 glimmer/20 motes for 5 weapon parts