I have hacksaw blade pieces on all of my firestarters that didn't already come with good ones, primarily the magnesium and ferro variety. I use the toothy edge on the magnesium. The back edge gets squared up on a file and that gets used on the ferrocerium rod.

Anything with an edge that will scrape off some of the rod when scraped against it fast enough will throw sparks. Broken glass works very well, especially if you can actually find a piece from a thicker bottle. If I need to mention to anyone to be careful handling broken glass, then those folks shouldn't be setting anything on fire or even on the internet.

As to the original topic of the thread, I have two of the large "army" models. I put the "'s in there because they weren't touting that connection when I got mine, I just knew that I had a choice of large or small and I picked the large model for the greater shower of sparks. I haven't had a single complaint so far over several years. I've had some ferro rods turn to dust in a week it seems and others that hardly corrode at all (at least in the KY/IN/OH area). I check the one thats packed whenever I check the pack and it still looks fine.