I had asked this very same question some time ago specifically regarding the 30-30. With the help and advice of members here and elsewhere I came to the conclusion that a 20-28 guage is about perfect for the 30-30 depending on the maker and size of the donor action. I start gunsmithing school on the 23rd of August and plan to build this very gun as my project in the 2nd or 3rd semester. I have consulted with the department director and I plan to use a Stoeger action as the donor. I will look at the 20 and 28 guage side by side and decide at that time which to use. We are required to build a BP gun first but we can do anything we wish after that.