A much easier way to use magnesium is to buy it in shreds form from a chemical supplier. Over here in the UK it was about $10 for 250g ( about 8 or 9 ounces). Seal amounts in a plastic drinking straw as you do cotton wool balls. When you need it, tear the end off and pour out the contents, add a suitable preprepared tinder pile and you have fire. I've used it this way several times and it's a lot easier than scraping it off a bar and the heat generated has always started a fire. <img src="/images/graemlins/grin.gif" alt="" />

Forget magnesium bars.

Let me tell you the fire you need to think about: It's 7:00 PM, dark, November, steady rain, 35 deg. F, 20 mph sustained wind, you're freezing cold - if you can start THAT FIRE all the others are easy! There won't be a dry ball of grass anywhere - unless you brought it with you - and it won't be dry for long! So, here's my system:
- vaseline soaked lint or cotton balls (I'd want some at least size of golf ball)
- good sized ferro rod (you pick but go for a substantial one
- hacksaw scraper
- split down wood to get to dry center (crouching under poncho!) - look for birch, cedar, fat pine (or whatever in your region) - use the bark, too - sticks size of a pencil or smaller
- don't stop splitting until you have a pile "as big as your hat" (Horace Kephart)
- take three of the sticks and make a pile of shavings
- stick in vaseline ball
- strike fire and jump back!

If you need motivation read Jack London's "To Build A Fire"

Just my two cents!