I got my bolt back today…..I assembled everything and when I pull the charging handle back it only allows the bolt to move 1/8” – ¼” backwards. This is with the hammer in its resting place….I opened the rifle and cocked the hammer by hand then shut. I could then pull the bolt all the way to the rear with no problem (repeated numerous times). I have removed the bolt and held it on the lower to see if anything has changed…..Any ideas…..I have a RRA lower and a RRA 9mm hammer.

I put the upper on a 223 lower (.223 hammer) and it worked fine. The RRA 9mm hammer has a ledge in the hammer face slightly above where the hammer would meet the bolt catch. I am thinking that this may be the issue. Should I get the dremel out?

Thanks in advance....

I honestly don't understand why the bolt won't push the 9mm hammer back but one of the reasons for getting the bolt ramped was to eliminate the need for a 9mm hammer. You should be able to put the .223 upper on the lower with the 9mm hammer and be able to operate the charging handle and cock the 9mm hammer with no problems. If the .223 upper has the same problem as your 9mm upper then I would look into other causes for the problem. Try ordering a standard AR hammer like the kind in your .223 lower and replace the one in your 9mm lower with it if the .223 upper works ok on the 9mm lower.

By the way -- is this the profile of the 9mm hammer you have?

if it works with the .223 hammer i would just go pick up a new hammer

Quoted: I got my bolt back today…..I assembled everything and when I pull the charging handle back it only allows the bolt to move 1/8” – ¼” backwards. This is with the hammer in its resting place….I opened the rifle and cocked the hammer by hand then shut. I could then pull the bolt all the way to the rear with no problem (repeated numerous times). I have removed the bolt and held it on the lower to see if anything has changed…..Any ideas…..I have a RRA lower and a RRA 9mm hammer. I put the upper on a 223 lower (.223 hammer) and it worked fine. That  was the reason to ramp the bolt -- M16 would take a beating to the hammer pin from the original profile on the 9mm bolt, so somebody took and cut a ramp to mimmick that seen on the std M16 carrier... Lose the auto sear lug and now we have a std AR15 hammer.   Put a std un-notched  hammer in it and enjoy your new upper. The RRA 9mm hammer has a ledge in the hammer face slightly above where the hammer would meet the bolt catch. I am thinking that this may be the issue. Should I get the dremel out? NO, lose the 9mm hammer, it was a waste of $15 in the first place. Thanks in advance....

The 9 mm hammer is shorter than the 5.56 hammer.  The 9 mm hammer looses contact with the bolt while cocking when the bolt has been ramped.

Use a  5.56 hammer with a ramped bolt.