This mod can be purchased unranked from the Void Trader Baro Ki'Teer for 220,000 220,000 and 400400. Note however that Baro Ki'Teer's stock changes with each appearance, and may not have this item available at every time.
Unlike most Primed mods, Primed Pistol Gambit offers less critical chance per rank than its normal version, giving only 17% per rank compared to Pistol Gambit's 20%. However, Primed Pistol Gambit has a higher maximum rank along with a lower overall mod cost for similar benefits.
At the cost of needing more Endo in order to upgrade this mod, Primed Pistol Gambit boasts a better critical chance to drain ratio.
For example, at base rank, Pistol Gambit supplies 20% critical chance for 4 drain while Primed Pistol Gambit at rank 2 grants 51% critical chance at the same 4 drain.
Had Primed Pistol Gambit followed Pistol Gambit's format, it would have had 220% critical chance at the cost of 14 drain.