A A2 rifle buffer weight is about 5.2Oz . A 9mm buffer is a wee bit more at about 5.5Oz
Therefore, if its not cycling properly with A2 , a 9mm will not be any better
What loads are you shooting ....115gr , 125 gr ?
Just to clarify, gun does not strip off another round and does NOT lock open ? or it does lock open on empty mag
Because if it does lock open, its not your load / buffer etc because bolt is travelling back far enough
It is a mag block adjustment generally that is the case. Depending on your mag block ( Hahn is the best BTW ) some are finicky on where they sit
I have had a RRA where if you didn't push it up as far as possible before tightening, it would not strip a round off reliably on some magazines
One of my lowers has Timiny 667 ( 3# ) it has not problems in my 9mm . Your 670 has even heavier hammer spring . No reason for light primer strikes due to trigger
Further to that, I shoot lots of 9Major ( over loaded 9mm that pushes 124gr @ over 1400 out of a pistol) that requires small RIFLE primers , which have harder cups. I never get light primer strikes with any of my lowers, all of which have less than 4 lb pull
I would check your firing pin protrusion . Only time I have seen this in ( no in AR ) is primers set too deep. Compare your reloads to some known working rounds