9mm AR-15 hammer.

[hatchetbearer]

I was looking at the spike's tactical 9mm uppers over at AIM, and it got me thinking about getting one, but use the same lower instead of build a whole new rifle, I know in addition to swapping uppers, id need to remove the mag block, but will the 9mm hammer function with a 5.56 upper?

 

[c5_nc]

You don't want to use the 9mm hammer. You want to buy a ramped bcg (several vendors sell them) or send your BCG to ADCO to be ramped. Then you use standard hammer with both uppers. This is what most people do now, even if you have no need for anything other than a 9mm AR.