I've wondered about that too. One application that I can think of is placing a long rifled insert into one barrel of a shotgun so you could have a deer rifle/shotgun combo. You could also make a cape gun or a double rifle out of a shotgun for a small fraction of the price of a real one. Because ot the way double barrels are set, aiming such an altered shotgun as a rifle would be a real challenge, and would probably require a scope and a bead. Some states require trappers to carry a .22, but some trappers would rather have a shotgun. To avoid lugging two guns, they can make a combo from an inexpensive double barreled shotgun with a .22 rifled insert in one barrel for $90. Many cartridges use a bullet that is nearly identical in diameter to other cartridges. You could use a chamber insert to adapt a magnum rifle to shoot a much cheaper standard round for plinking. You could also ressurect some old military guns with a chamber insert. Accuracy could suffer, sometimes the inserts get jammed in and require a gunsmith to remove, and copper/lead fouling and excess barrel wear can result. A person would really have to know what they are doing (not me) to avoid damaging their gun at worst, or just wasting money at best. I have found that if I save my money, I can always squeeze one more gun into the locker. Since you can buy a new .20ga/.22 combo gun for around $350, I'll pass on barrel inserts.