Using a New Frontier Armory lower with a mil spec metal hammer, a CMMG 9 mm BCG my pistol build will not set the hammer when charging.  Any help will be greatly appreciated. The hammer I am using is a Delton. I have read other posts that mention using a DPMS hammer.  I will post photos if it helps.  Thank you.

Is the new frontier a polymer lower?  I had trouble with the ATI first gen doing the same thing.  I think either the holes are out of spec for the trigger pins, or the lower flexes.

I tried 5 different fire control groups (all from working rifles) and never got mine to work.  Sent it back and ATI replaced it with 2nd gen "hybrid" lower that worked.   Interesting enough, there was more material around the trigger pin holes making me think that was to address this issue.  Would recommend the samething.

Interesting.  I am on my 3rd hammer change out.  I did the same thing.  Almost as if the bolt is not pushing the hammer down far enough?

Have you checked the disconnector, and disconnector spring yet?  Did you use the correct spring in the lower parts kit for the disconnector?

It is common to mix up the bolt catch spring in place of the disconnector spring.

I will check the spring. I started with a New Frontier Armory lower  it came complete assembled by them. I tried the upper on a dpms lower and it works. I can set the trigger manually   Just not when I charge the bolt.  Your advice makes sense. If the catch was a little higher? I contacted New Frontier.  I have not heard back from them.  Maybe I am the first to use one of their lowers for a 9mm pistol build? I am looking forward to taking this gun to the range.

New  Frontier Armory came through for me. They advised I need to install the complete metal trigger hammer set.  I did that everything works. Thank you for all of your advice.