If you are going to the trouble of making a conversion, then I would suggest find a resource for the dimensions of the cartridges and the power factors of each (might be important). A handloading guide should have all the dimensions you need to consider (would really be handy when checking someone else's work). The information would really be of service if you were going to try and create a "universal" conversion for all your favorite cartridges whose name begins with a 3 or a 9.