Double Shotted deals 10 - 13% of boss HP. Give or take depending on properties.
(Cataclysm Chaos spawn)
Which is still waiting on about 53 - 70 seconds of cooldown.
Damage and cheese only really ramps up when a concentration potion is involved and the player is competent enough to not miss and stun-lock the boss.
Huntsman can theoretically chunk 50% of boss HP in 6 seconds, possibly more, but it rarely happens due to his inability to stagger/stun-lock.
Piercing Shot can theoretically kill a boss in 15 seconds without a potion, obviously that’s not really possible unless a boss is sitting still. Despite that, nobody calls out that talent for being cheesy or OP.
It seems more like a stagger/stun-lock problem than cooldown.
FS haven’t explicitly said it’s not intended.
“Fire two powerful bullets in a straight line. Scoring a headshot with this attack reduces the cooldown of Locked and Loaded by 40%.”
Scoring a headshot grants 40% cooldown reduction. Double Shotted has 2 bullets; 2 chances to headshot. Plus, 1 bullet can potentially score multiple headshots granting more than 40% cooldown.
It seems more likely to be intended than not.