A Rolling Block is a simple fairly strong action that is simple to make and operate.
Here's a fair pic of an action.
http://www.gunbroker.com/Auction/ViewItem.asp?Item=64139247
To open it up you'd pull the hammer back to full cock and then pull the breech block open to expose the chamber. The extraction happens when the breech block is opened. When the hammer falls it locks the breech as it falls.