There are several bolts available to purchase for upgrading your Mini.  The real question is whether or not these bolts are really an upgrade from the stock bolt.  Many players report that after "upgrading" the bolts on their Invert Minis they had the need to tune their gun and many reported that they need to retune everytime they got to the field.

Most people upgrade their bolts because they are looking for better air effeiency or better consistency of air or to get a bolt that is easier on their paint.  Some people feel that the Mini a little rougher on paintballs than more expensive paintball guns.  A new bolt my help that.

Based off what I've read I probably won't upgrade the bolt in my Mini unless I have compelling reason to do so.  It's shooting fine and I'm not breaking paint so I don't really want to risk putting in an aftermarket bolt.