.355 vs .357-.358 depending on if it's jacketed or lead. The bullets will swage down assuming the Ruger even bothers to use different barrels. It's not unusual for a manufacturer to use the same barrel between their 9mm and 38/357 guns (and 22lr/22mag). The oft-suggested Federal match wadcutter would be perfectly safe being both low pressure and soft lead.
ETA: You can see nearly that much variation in 38 specials of different vintages. Slugging the barrel would cover off any concerns.
Chris