Bought an FCU and Parts kit, I put the SOB together just like all the videos say and the SOB won't fire. Any suggestions?

Sounds like the disconnector is working properly or sear springs are out place or...without pictures and more info it’s hard to diagnose the problem.

Which trigger are you using? I bought a factory flat trigger for my p320 FCU and it wouldn't fire either because of the overtravel stop on the rear of the trigger. Trigger wouldn't move far back enough to release the striker. I carefully filed a little bit off the overtravel stop until it would drop the striker, then took off a tiny bit more for good measure and to account for carbon buildup etc. Worked fine ever since.

Thanks I will give it a try