Beef casing are available, but are not what we think of as a casing. They are much larger in diameter due to the size of the animal,and they go by other names than just a "beef casing" to help define their size. Cows have a different digestive system than pigs and sheep, so the intestines differ and they are also not considered to be "edible" casings. They are thick, leathery, and do not stretch like the edible pork and sheep casings that most of us use. You just couldn't substitute a beef middle for a pork casing, they are vastly different in all manners.
If you are trying to avoid pork casing for religious or health reasons, there is the alternative of edible collagen casings, but they don't provide the "snap" when bitten into.
I use beef bung for capicola, these average 105mm to 110mm in diameter.
Beef middles are a little smaller 55mm to 60mm great for dry salami, summer sausage, and other dry cured sausage.
Some also use beef rounds.
It really all comes down to the type of sausage you are making. The best advice I can give you is to follow a proven recipe which also includes the type of recommended casing.
