I bought the Doyle Riv-Nut setter thinking it would be handy and make it easier to pull the 1-4 & up riv-nuts. What a surprise when my brought it home and we opened up the package and found no mandrel for the 3/8-16 riv-nuts. I am lucky that I hadn't had a use for the larger riv-nuts lately.
I found that the 3/8-16 mandrel kit can be bought of Amazon.
Here is the Doyle. It is really lacking in the mandrels. The Marson has more mandrels that come with the tool kit.
Doyle Kit Mandrels with kit: - #10-24, 1/4-20, 5/16-18
Marson 39314 Kit Mandrels with kit: - #6-32, #8-32, #10-24, #10-32, 1/4-20, 5/16-18, 3/8-16