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It's a exotic weapon. Who's says what can and can't happen. I think it's a great selectable option. You can have 10 in the mag or a possible 14 if you hit your precision shots. It works, most likely you would only proc two extra bullets most of the time.
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The perk tree is for stat modifiers. So you could have extended mag if you wanted, but not triple tap.
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Triple tap is more interesting because it's not a given 14 shots but a skilled return of 14 shots.
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Then it should be in the perk slot. The perk slot is for more interesting and active abilities, while the perk tree is for abilities that change the base stats of your guns
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You can choose to increase stability or reload at the cost of range or keep the set stats and go for a possibility of extra shots.