Right. But would it be possible to just grind some feedramps into the existing barrel extension with some careful dremel work?
Have you considered just selling that upper and getting a 16" upper in the config you want?
I have but this was the original upper for this carbine and holds lots of good memories of a buddy that is no longer here. I currently have 2 lowers but 4 uppers and I am working on a special project a SPR MK12 style rifle that I can use for 3 gun matches. I like to DIY so I try to get as much knowledge from people that know the subject that I bounce ideas and with their help I choose the parts run them by a couple of buddies then if I have not made any mistakes order the parts and have an assembling party. :)
just buy a flattop without the ramps.