I bought the 1/4 inch Tapcon screws at Lowe's, in a package that included the correct mortor bit.
Everything worked fine!! The screws went in fine, and seemed to "tap" or "seat" as I would have expected. I put three screws in each bracket mount for my shelves.
I didn't know about Tapcons, but now I do, thanks to many of you who gave me good tips.
Thanks to all !!
James