I don't have one but I've seen plenty and used one or two. Pretty much the standard for CSR matches now.
Barrels are much heavier and often free floating, without the need for a gas tube.
A variety of cocking handles have been used, some like the FAL one with a slot in the upper, some directly attached to the bolt carrier.
Separate uppers offer no advantage here, they would have to be registered like a complete rifle.