What Double Barrel Fits
Choosing a double barrel shotgun in the wing-shooting world is a personal preference, whether it be a side-by-side or an over&under. A lot of avid wing-shooters wouldn't shoot anything other than a side-by-side, as some think it fits the epitome in the wing-shooting world. Some people suggest that the over&under is more accurate than the side-by-side. Maybe with the over&under it's the single plane barrel they're sighting down that makes it easier for pointing, instead of two parallel barrels. I've had discussions with people who say that the over&under is better for leading your bird than a side-by-side because of the barrels being a single plane view. Their opinion being, you can get a better perspective on leading your target.
I had a client who owned a side-by-side,12 gauge Purdey. It had ivory sights with 30inch barrels and a prince of wales grip. It was muzzle heavy, as he hunted ducks and wanted the weight in the muzzle to establish a better follow through when leading the bird. It was a beautiful gun, and expensive, but it wouldn't be my first choice for the grouse and woodcock cover. This gentleman owns many double barrel shotguns, all of which are side-by-sides. He shoots a side-by-side preferring the way it looks and feels. Again this is all personal preference.
The only significant difference, other than the barrels, would be the fact that over&unders are single triggers, and side/side's can be double or single trigger. I have a side-by-side 20 gauge Churchill that is a double trigger, and it's awkward to shoot after I've used my over&under 20 gauge with a single trigger. Keep in mind, both designs are going to shoot the same, as you'll still be sighting down the center of the barrel.
Either one can have pistol grips, prince of wales grips or the straight english grips. The vast majority of over&under's I've come across have pistol grips. My over&under has a pistol grip and for me it's easier to carry through the brush. Particularly, when I'm running my dogs and having to hold the gun vertically and one handed. The pistol grip design contours to my hand, making it easier to maneuver the gun though alders. My 20 gauge Churchill has a prince of wales grip, and is essentially the same as a pistol grip, but with a more open curve. Some people prefer the straight english grip. They find it more comfortable to carry through the woods, and when mounted to the shoulder with two triggers it's a smoother feeling going from the front trigger to the back, as opposed to the pistol grip and wales grip.
In choosing an over&under or side-by-side remember they both serve the same purpose, but the key is to select the style you feel most comfortable with. Both models and styles offer advantages and I consider myself fortunate to have had experience with both.