nail polish is harder to do in Sheridans because of the fixed barrel.
As stated Palmers does wedgits. They're basically little dents in your barrel that prevent rollout. It's 25 bucks plus shipping.
There's also a cheappo way to prevent rollout. It doesn't look great, but works. You can take yard and let it go down your barrel. Attach it through the feed tube. It'll let your ball fire, but hold it snug. You'll need to replace the yarn throughout the day of play because the bolt will chew it up.