After seeing some threads here about these things and the groups I'm thinking about having one built. 1. What are the common/adequate systems (receivers etc)? 2. What chambering? 3. Do they require different load data than a rifle in identical chamberings? 4. Who makes a good one? 5. What is the cost? (I don't think they'd be much different than a rifle overall, same parts just shorter.) 6. When can I get one?