As above I prefer natural casings but be careful and make sure you buy quality, soak them over night starting with warm water when cool put in the fridge, rinse in cold and run them out of a tub of cold water getting some water inside the skin before you start spooling and it wont split and suggest you start with thick casings 30mm ish until you get into it. Worked in the meat trade for many years and we always bought from https://www.weschenfelder.co.uk/ They are quick with delivery and don't take your eyes out.You wont go far wrong with this outfit, their Royal venison season mix is spot on when 20% fatty pork is added (Belly pork is good) and if you like a bit more heat the Yorkshire traditional mix gets you sweating. Good Luck
T