I learned something today in Fallout new Vegas i was not aware i could do it. Lets say you buy shells for the Riot shotgun, you can buy diffirent types of shells like Slugs, Bean Bags or just reular buckshells. You can switch from regular to Slugs or any other types of shells you purchased for that weapon, by pressing the anolog up button on the PS3.

I always wondered what is that damn weight i'm carrying. Guess now i know, huh?