@kingpins and @310843B and whoever else might be interested in sausage casing general knowledge.
20mm casings are generally the thin sausage skins, can be called Devro's, Nippy's and a few other names, they factory made skins which contain a lot of collagen and are also known as collagen casings. Theres no gluten in these collagen casings or natural casings. . Other thin casings, around 20mm are sheep casings, these are the natural ones, naturally grown as nature intended, obviously these are lamb/sheep intestines, washed sanitised and packed with lots of salt, these skins are much more expensive than the collagen skins but most people would agree they are a better product.
30mm, 32mm or around this area is the thick skins. Can be Collagen skins or natural, the natural ones for thick sausages come from pigs, the price of these are not so bad and might even be cheaper than the collagen skins from memory if bought in bulk as a butcher would buy them, i've always bought the thick natural ones rather than the thick collagen ones and this would have been due to price.
If one has an automatic sausage making machine then natural skins can't really be used so shops with that machinery will only use collagen skins because if one used natural skins with that machinery an extra staff member would be required to fill the sausages and the job would take longer than if you just used the old school hydraulic or water fillers, it basically defeats the purpose of having the sausage machine thus collagen the go for them.
Not exactly sure why humans have developed gluten issues over the last 30, 40 or 50 years but there will be a reason and these gluten issues have changed the sausage meal ingredients. Most sausages are now made with rice flour rather than wheat flour, the wheat flour has the gluten. Most butchers if not all will not use or do not use wheat flour sausage meals anymore because it's just not worth the trouble and potential legal risk, it's a lot easier just to make them all gluten free.
Pepperoni casings, salami casings, shoulder ham casings etc use different type casings to sausages in the commercial world, they are a fibrous casing made out of some sort of fibres, they come like in single skins say about 30cm long and maybe a couple inches wide, a bit wider than thick skins, they generally come with a string crimped on one end with the other end open for filling, these casings have tiny pin like holes along the length of them to let any trapped air out so no airlocks and once filled the end gets crimped off.
People often come into the butcher shop to buy skins if you don't want to get them off ebay.
I would recommend the collagen skins for home use and novices and if your only making a few kg's.
