The way I have interpreted the Deed "Target torso". Is that when spending 1 grit and a full round action to have a critical threat of 19-20. Thus, the effected weapon becomes a 19-20. Since all firearms are known to need a natural 20 to crit, this deed effectively causes the firearm to naturally become a 19-20 crit threat weapon, not increasing not expending, even though the effect is literally improving their critical range.
However, this is balanced by only one shot since full round action is required as well as spending one grit point. Therefore, it does stack with keen,improved criticals etc. Just to restate my interpretation of this 'Deed' is that it enables to weapon to naturally become a 19-20, thus not technically being modified by enchantments or feats. Where Enchantments is a direct modification and feats are a modification on the character. The 'deed' is an ability that allows the weapon to naturally become a 19-20 crit weapon.