I have a 1978 Browning Catalog featuring their, new at the time, Browning Continental that stands as testament to how long I've wanted to get a double rifle. I am very much a bird and duck hunter but I do meat hunt for deer. I purposefully waited because, in my opinion, anticipation and long-term goals keep us mentally youthful and more interesting as people. Anyway, when I finally decided it was time I just couldn't come to grips with getting a Continental. I have a slim-waisted 20 gauge side-by-side that is my go-to bird gun and it's IC/M barrels are much more useful than the Continental's M/F barrels. Plus with most of the Continentals I would still have to get it fitted for sling studs and a scope rail. Then, I found a Browning Express with scope rails, swivel attachments, and, what I learned here, a cartridge carrier so I jumped at it.
I knew it was a roll of the dice to see how the barrels shot relative to each other but all I could do was plan to deal with what ever differences there were. My deer hunting is usually in the Southeast woods so I don't need a 200 yard rifle. The longest shot I've taken in memory is about 80 yards. Anyway, after shooting more than two dozen rounds through the "new" gun I found the lower barrel shooting about 1.2 inches left of the Over barrel at 100 yards. The windage differences are actually much better than I expected. The Under barrel, however, was averaging 2.8 inches lower at 100 yards - close to unacceptable. Then I put a 180 grain in the upper barrel. The elevation differences disappeared. Right now, it only has a 1.5-5x scope but if I load two 150 grain bullets I have a under barrel that shoots a tad low at 100 yards and an over barrel that shoots point-of-aim out to about 225. I'd never take a deer shot that far even with a higher mag scope but I did kill a 200 yard coyote last year and have a beaver that will need killing if I ever see him from the stand. With a 180 in the top and a 150 below I'm point of aim to about 125 yards with a choice of which bullet to start with.
Here's a 100-yard four-shot group that, from left to right, is under, over, under, over. I may still move it 3/4" to the right but I think I've got a 100 yard rifle that will kill a deer every time I do my part. BTW - I knew I pulled the first shot a tad and thought about starting over but more folks were showing up at the range so I just wanted to finish up and get out before they started shooting.