As moderator of Gun Owning Atheists, I'd like to recommend you consider offering your group a discussion on the difference between support for US citizens owning guns and support for unrestricted US export of guns. This article about the International Arms Trade Treaty claims the the US is the biggest player in International Arms trade.
... [The] international Arms Trade Treaty ...would regulate the largely uncontrolled world of international weapons exports while still leaving domestic laws untouched.
With its illicit and indiscriminate sales to non-state actors and warring factions, the global arms trade has been declared one of the three most corrupt businesses in the world. In countries such as Somalia, Yemen or Syria, weapons and ammunition are regularly sold to known human rights abusers,...
As the largest profiteer of the trade—accounting for over three quarters of global sales—the U.S. has a particular responsibility to make sure weapons don’t fall into the wrong hands.
While you might not personally question the NRA's position that this treaty threatens US second amendment rights, this is a serious moral issue that should generate much thoughtful debate among Atheist gun owners. Honest gun owners who despise the illegal trade of weapons to individuals and groups considered US enemies may make up a substantial portion of your membership.
In response to your question about my George Washington quote:
It is from the "Treaty of peace and friendship between the United States of America and the Bey and Subjects of Tripoli, of Barbary," most refer to it as simply the Treaty of Tripoli. In Article 11, it states:
"As the Government of the United States of America is not, in any sense, founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquillity, of Musselmen; and as the said States never have entered into any war or act of hostility against any Mehomitan nation, it is declared by the parties that no pretext arising from religious opinions shall ever produce an interruption of the harmony existing between the two countries." Link to full text