Looks like it's a 1/12 chance to get it. I'm still pretty sure it's rng but something you can do to help with shards is get alot of glimmer, buy a heavy telemetry, buy any heavy from the kiosk, use your heavy telemetry, use 6 motes of light (this was the last number I know) and then dismantle the gun and you should have enough xp from the telemetry and motes to get 2 shards when you dismantle it. I hope it helps