You should be able to find customer service information here:

www.gun-tests.com

I'd try dealing with the folks at the magazine rather than some scum working for a collection agency. From everything I've heard, Gun Tests is a reputable group. I find it hard to believe that they're trying to scam you. Explain to them that you've never received any of their magazines, and ask them to produce a subscription card or something showing that you ordered the magazines.

As someone above said, unfortunately, it might be worth it to just send in the $24 to get rid of a headache. It'll cost WAAAY more than that to have an attorney deal with them. And, your credit rating is worth a whole heck of a lot more than disputing $24...