Theoretically, an U/O double rifle should be easier to regulate since the vagaries of muzzle flip do not come into play - but so few U/O's have been made by Gunmakers that cared to spend the time to regulate them properly that it's hard to know whether this is true in practice.

Witness the Winchester 101 U/O's that were "jig regulated" - NONE of them shot and probably put most shooters that owned them off the concept for life!

Vic Pedersen, the Darwin gunsmith had a small sideline going in re-reguating them - when he finished with them they usually shot brilliantly.

Cost has got to be an issue - any Gunmaker making both configurations will alway list a "stackbarrel" at a premium over it's vertically challenged siblings.

Leaving aside the issues of gape and cost it really comes down to personal preference as it does for shotguns.

For example, a shooter that is seeing large slabs of barrel on an U/O with his left eye, could well have an eye dominance problem?

Comforting to know though that the U/O double rifle is held in such low regard, that if/when I come across my personal "grail"; a Boss O/U double rifle, preferably in .470NE, none of you blokes will want it!

P.S. For those that missed it - the Boss is the only true O/U - all the other "superposed" jobs are U/O's, since Boss held that only their action was strong enough to set them to shoot top barrel first!