They are quite bulky - one of the bulkiest ways of starting a fire I know. Much bigger than a Bic or a Sparklite or matches or whatever. Mine has a danger of drying out and you have to keep the seal lubricated (I use vasaline), so it is not really an ever-lasting device even if you can improvise tinder.

If I'm somehow incapacitated and a stranger is rooting through my equipment looking for things that will help me, and they see the smooth wooden tube with the pot of vasaline next to it, "firelighter" may not be the first use they think of.

Hmm...like a little diesel engine in your pocket, or is that a ramjet?

Anybody make these out of metal?

There is a website on firepistons (of course there is), it will probably tell you more about the things than you ever wanted to know...

Any sticky fat will do. When I first read the description of these things in a book, it was stated that dog fat was used where the author first saw them.

And yes, it is about as bulky an item as you can think of for firelighting. And magnifying glasses are about the slowest. I don't think it has a place in a PSK or even a smaller pack, but in a larger pack, the smaller polycarb ones might have a place. And as a long term household item, if you have one, you're set. You don't have to reinvent the firebow to conserve matches. :P

Never seen an all metal one, but I've seen them that were part metal and part or fully polycarb.