Well, the JPFO has been supporting RKBA for awhile now. I don't know membership numbers but from things I've read is that it is probably 20-30% Jewish. Supporting RKBA isn't even a new thing among the Orthodox community.
If I was a Jew in America I would be arming myself as well (okay, I would have done it a long time ago anyway just because). We have been seeing a growing amount of anti-semitism. Just look at signs appearing amongst the Occupy movements. Probably even more disturbing is how it seems to be growing in acceptance especially amongst the liberal intelligentsia. Perhaps a lasting part of the Obama legacy will be how his regime drove Jews out of the Democrat party. They have been trying to move the US from being sympathetic to the Jews to the Palestinians. I think those sentiments then also become felt domestically as well. The left continues to try to remove all Judeo-Christian while showing increasing tolerance for institutional acceptance of Sharia law. So yes if I was a Jew in America, I would be very concerned.
Need to bear in mind that, while linked by ties of faith and family, an American who happens to be Jewish, whether merely by descent or observance, is a very different person than an Israeli, whether he is Jewish or not. Israel is a POLITICAL entity. Judaism is a religious faith. And the people we refer to as Jewish are tied by an ancestral ethnicity, even though they now come in all shapes and colors. How these three very different things interact with each other, and the rest of the world, is a never-ending source of fascination.
Back on track, though, if I were an American Jew, i'd be armed to the teeth just on principle, given world history.