I found this post which seems to hit the nail on the head.... Last couple sentences really sums it up:
KNS pins are not recommended on the Geissele triggers. The trigger pin doesn't matter, it's the hammer pin. The assembly uses a circlip INSIDE the hammer pin bore to retain the hammer pin instead of the conventional J spring, and the hammer spring leg retains the trigger pin (as original) in a groove in the pin.That said, you CAN use the 1st gen .155 KNS pins. Geissele has said that it will overstress the circlip and eventually wear out it's tension, thus your hammer will be married to the KNS pins for life.2nd gen pins change the hammer pin to a solid, with the little tabs to retain the outer bars. The 1st gen hammer pin is hollow, with oil holes, and uses a little screw in 'bullet' to aid in insertion. It will expand the circlip and allow installation. No such luck with the 2nd gen, as the sharp sholder will damage the circlip, and likely force it out.I used 1st gen pins in one of my SBR lowers, which I intend will stay in it's current configuration for my lifetime, so nothing lost for me. My other went into another SBR lower that I'm still messing with, thus the factory Geissele pins.KNS pins, contrary to inter-rumor-net, will not in any way ruin the operation of the trigger. *Geissele has noted in several emails that they will replace the circlip, for a fee, as long as the bore/retaining groove has not been damaged.So, it's up to you. 1st gen pins and possibly stressing the factory circlip, 2nd gen pins and probably damaging the circlip, or Geissele factory pins. In any case, use the larger diameter pins.