Thing is Topph can Metalbend any metal in her line of sight, So she could wreck his weapos too.
Problem... Metal Bending doesn't actually bend metal, it bends the Earth Impurities within the metal.
1) The Staff and Zat are made of an Alien metal, and I would think would be Exceptionally pure, and therefor Uneffected.
2) The Metal parts of a P90 are to Military Grade, and I would think of exceptional quality too considering these ones are given to teams ventureing off world. While probably not as pure as whatever metal Jaffa weapons are made of, They would still be of an exceptional purity to the point where they wouldn't be able to be bent.