The ownership rate reported is the average estimate taken from "Annexe 4: The largest civilian firearms arsenals for 178 countries. That table gives also the minimum and maximum estimates. Note that for some countries, this margin of error is considerable. E.g. Yemen, ranked near the top with an ownership rate of 54.8, has a low estimate of 28.6 and a high estimate of 81.1. While the United States is ranked for the highest gun ownership rate unambiguously, Yemen based on the margin of error may rank anywhere between 2nd and 18th, Switzerland anywhere between 2nd and 16th.
Morgan says that Yemen has the second highest gun ownership rate, according to Wikipedia there can be a great margin of error in reporting.