whatever trigger PSA puts in their 9mm lowers. It is the deepest light strike I have ever seen. Tried a few brands of ammo with no success. Found another person that had the same issue and swapped firing pins and solved the issue. I ordered a main bolt refresh kit from Matador, hopefully that works. link to other light strike guy. I didn't notice any obvious defect on my firing pin but I'm no gunsmith.
https://www.reddit.com/r/AR9/comments/xvmtm2/matador_arms_montgo9/