I’m a leftie with left dominant eye, so the rifle will be on my left shoulder whenever possible.  From what I could find, Stag and RRA make left hand uppers, however RRA cautions their LH uppers only interface with their lowers. I believe Stag only has LH carbine systems.  I'd like to build a LH middy. If I understand what I've read correctly, Stag uses a proprietary ambi barrel extension with their LH upper halves (maybe their std one also?). If I buy their LH upper receiver/BCG, can I use a std extension with barrel of my choice?  Has anyone done this with 14.5/16 middy?