Howdy,
I've got a 6520 Gov't carbine(carry handle, 1/7 twist, 16" pencil barrel and I bought a 6721 upper ( 16" heavy profile, 1/9" twist and flattop) to use optics.
The 6520 is a joy to carry, but jumps more in rapid fire than the 6721. I really like the muzzle heavy feel and aid in keeping the muzzle down with the heavy Bbl.
I'm sure I'm a little biased as it wears an Eotech 552 as well.
Also, I've shot better groups with the heavy Bbl and attached iron sights than with the light barrel. Both started cold.
FWIW.
Gary