cullbuck

Why on earth are double-barrel side x side shotguns so much more expensive than over-and-unders?

Pygmy

In guns of equal quality, they are not...

In high quality guns, both require a considerable amount of hand fitting and tuning and high quality guns of both types bring premium prices..

Perhaps there are just more cheap, mass produced O/U guns available than there are cheap, mass produced side by sides...

I suspect that this is because the DEMAND for O/U guns is higher than it is for side by sides..

Wingbone

Both barrels have to hit the same spot. This requires putting the gun together, shooting it, taking it apart, making adjustments to the fitting of the barrels, putting it together again, shooting, taking it apart, etc. until they both shoot together. In the best guns, it takes time and skill to do that.