We have a 20 bore B Rizzini, and it has given good service for last 15 or so years. Its a nice solid little gun that just works well. No its not a high grade Berretta, Rizzini, Ceasar Guerruni etc but then I only paid Ł250 for it 2nd hand.

Most of the Italian gunmakers do make over and under doubles and they generally shoot reasonably well. Is the one you are looking at an 8x57 or the 8x57R - ie it takes the rimmed version. With a break action and rimless cartridges, you have a little plunger on the extractor that goes into the groove on the cartridge and you have to pull the empty out. With a rimmed case you just tip the barrels up and they fall out.

The 8x57 and the 8x57r are both popular in Europe and good for all game.

With an over and under you will find that one barrel shoots to point of aim, and the other a bit higher. For the 100 + shots i would treat it as a single shot and use the barrel to which the sights are aligned and shoots best. The double barrel is really a shorter range rapid follow up for driven piggies type affair. Here a 2 or 3” group at 50 is more than adequate.

With any double where barrels are soldered together, you will find that as the barrels get hot shots start to wander. This is due the barrels not be able to expand evenly. With a double they are usually regulated so its a bang, bang rather bang wait a few moments and then bang for the other barrel. Ie two quick shots go close together.

I would expect it would have been regulated with factory ammo. Main European Brands will be Sellier & Bellot, RWS or Norma. I would look at one of these, and try and replicate a similar bullet weight and velocity. I would at the older and heavier bullet styles - something along the lines of https://www.sellier-bellot.cz/en/products/rifle-ammunition/rifle-ammunition-sp/detail/128/ as opposed to a modern ballistic tipped non toxic bullet.

I suppose a lot depends on the price. If the seller is holding for big money then pass, but if its a good price then why not.