I want to get a 16" upper for my preban olympic arms lower. Right now the upper I have on it is 20" flattop with freefloat tube and rail system and it is heavy. I have been looking at the RRA midlength A4. Is this a good combination-Olympic Arms and Rock River? Also, as far as a flash suppressor- I am not sure if I have this right. Can I have the barrel threaded and have a removable flash suppressor as long as the barrel is 16"? But if the barrel is cut down and has a flash suppressor attached to it to bring it up to the 16" the suppressor would need to be on permanent. I want to be able to have the freedom to remove the suppressor without any difficulty.
              Thanks

You should have no problems with the RRA upper and the Oly lower. You would have to keep the barrel at least 16" if you want to remove the flash suppressor.