I made myself an iron-sighted play-thing a few years ago on an AR platform.
I took a 28 inch blank, 1-8 twist, and turned it to standard rifle contour from the gas journal to the breach. I then turned the barrel from the gas journal to a point 3" behind the muzzle at .680" From the point of the .680" diameter 3" behind the muzzle I then jumped back up to .750, and lastly I turned a stub 1/2" diameter and threaded it for 1/2X28 threads.
I mounted a low profile gas manifold on the barrel at the standard position for a 20" rifle and then mounted a GI front sight tower on the barrel up at the front just .050" behind the flash hider. I installed a hoop with a cross hair for the front sight itself, and I am using a standard GI detachable rear carry handle.
I made my own free float tube that come all the way out to the front sight tower. It would work just as well with a standard rifle length FF tube, but I like the way mine looks better. (It looked better before the pint got chipped off, but that's what happens when you carry them around a lot and get in and out of trucks with them all the time.)
This rifle upper has a 33-1/2" sight radius. That combined with the cross-hair front sight makes for a very good play thing. I will not say it makes me the marksman I was in my 30 or even my late 40s, but I can hit very small targets at 300, 400 and 500 yards with this upper.
Form a rest I hit 6 out of 6 chicken eggs with this rig at 240 yards.
Makes me feel a bit younger when I do that.