I've used AmmoSeek a few times, and it's definitely a handy tool if you're looking for specific ammo. The way it aggregates prices from various vendors saved me quite a bit on .223 and 9mm rounds recently. I found a couple of sellers that seemed a bit sketchy at first, because the prices were really low, but when I read reviews on them, they checked out fine. Just be sure to read the full product descriptions and watch out for shipping fees; they can sometimes creep up and counteract the savings. Overall, I'd say it's a solid site for comparing prices, just stay vigilant like you would anywhere else online.
