I shoot nothing but cast bullets in my Marlin 45-70 Guide Gun and Ruger Alaskan .454 Casull. I shoot gas checked bullets in the 45-70 because of the higher velocities and also in the Casull. I don't worry about gas checked bullets when I load the lower velocity rounds for the .45 LC which I also shoot in the Ruger Alaskan. Penetration with a 525 gr. Pile Driver shot from the 45-70 is incredible and this is my prefered load for big bear protection here in Alaska. I buy my cast bullets from BearTooth Bullets down in Idaho. Here is the link to the BearTooth web-site:
http://www.beartoothbullets.com/
If you decide to place an order, give Marshal at BearTooth a call first and he will make recommendations for your particular gun. The best time to get him on the phone is in the morning around 9AM Pacific Time. Most importantly, ask him about bullet diameter.