Yargh.
A "cannon" is a weapon.
A "canon" is a rule, and by extension (mostly thanks to the Bible) a collection of documents or other records that have been examined under such a rule and judged official or sacred.
This has been your Internet Spelling Pedant post for today.
Darn you beat me too it. I was going to show the difference with pictures.
(an old cannon vs. Jar Jar)