Call someone a "poor" enough times, you're bound to offend them.
Why should one be offended at that any more than at being called "rich"? Having money or not having money is no reflection on your virtue, which is the only thing of real worth. Poverty, however, does really limit your options in life, and that often includes defensive options. Also, many people are foolish enough to judge a man by his income, and so the fools won't have respect for you.
I do have a word for someone who buys guns for purposes other than collecting, has no knowledge of gun handling, and doesn't intend on acquiring that knowledge even when he can easily afford it: irresponsible.