[One constitutional right] is now so rare that it has become practically sui generis–namely, the right to bear arms. The only other constitutions in the world today that still feature such a right are those of Guatemala and Mexico, while the Argentinean constitution contains a somewhat different duty to bear arms in defense of the fatherland.
From David S. Law and Mila Versteeg, "The Declining Influence of the United States Constitution" (September 10, 2011) at page 34.
The percentage was about 10% of all constitutions shortly after World War II and has steadily declined, due to both repeals of existing rights and the formation of new countries with constitutions that omit these rights.
Of course, the text of constitutional provisions and the reality of gun control in a particular country differ greatly. Mexico, in practice, has a highly restrictive gun control regime despite having a constitutional right to bear arms. Switzerland and Israel have routine private ownership of automatic weapons, as a consequence of extremely inclusive militias in their nation defense strategies, but neither has a constitutional right to bear arms.
In the United States, while the constitutional right to bear arms that has recently been intepreted to create an individual right to bear arms applicable to both the states and the federal government, the scope of the right has been interpreted quite narrowly, so only one District of Columbia municipal ordinance at the federal level, and a handful of local ordinances at the state and local level, have been determined to abridge the right.