Guns, Dogs and Blades QnA

I'm not going to be much help and you probably know this but I'll give my 2 cents. I bought some steel field loads by accident last month for a pheasant hunt because it was the only #6 on the shelf. Didn't pay attention to the label. I wasn't sure how it would pattern, so I left it at home and used 35 year old #5 and #4 shot Remington Express loads in my Peerless over/under. #5 in the bottom barrel with IC choke and #4 and Modified for the longer follow ups. Combo worked well. If in your situation I would be inclined to go with the bismouth rather than steel for any gauge. It patterns and hits the closest to regular lead shot. If you can get to a patterning board to figure out what choke to use and shoot some trap to figure out the velocity and how much to lead them, then I would be temped to use the steel. Steel doesn't carry energy like lead does so you need the bigger shot which makes for thin patterns in standard length shells. I don't have any experience with tungsten.
I shoot ground squirrels in northern CA and understand your pain. The lead free bullets are expensive and don't perform as well.