This question comes back every so often. I decided to try collecting list of trigger groups that work with HK firing pin safety, so can be used either in lower other than HK or dropped into HK lower. Looks like most M16/M4 select fire triggers work with it, so list is of semi-auto triggers. Please take in mind that HK firing pin safety takes some hammer energy to disengage. Using very light trigger setup with light hammer springs can lead to light primer strikers on some brands of ammo (surplus, Russian, etc.):
ALG Defense ACT
Armalite National Match 2-stage
ATC AR-Gold Trigger (needs fitting for MR223 due to rear receiver pin location)
CMMG Gen2 2-stage
JP Competition Trigger
JP EZ Trigger
Geisselle HK MR556
RRA National Match 2-stage (CAUTION: Only new version with rounded hammer is compatible)
Timney AR-15 Skeletonized
UHL Trigger (German made)
Wilson Combat TTU-3G and TTU-M2
I will try to update this list when new info will come in. Please give confirmed info on other triggers, I will update list.
WARNING: Some triggers will not work properly with HK safety from 2-stage "European" trigger (one that allows to be put on safe with hammer down). Old style HK safety or aftermarket safety may be necessary.
Geisselle SSF - standard model reported not to work reliably with HK416, special model required.
Last update 19th of Sep 2016 (TTU-M2 added)