Lycanthrope, I had a 172 with a 180-hp until a wind-burst ate the hangar where it was kept--the insurance company bought the remains, and I haven't gotten back in the money-drain bidness.
I played around a bit with trying to fly three feet off the runway and not touch anything, which took a bit of concentration. Anytime I touched down, I did it slowly: 100-knot touches are hard on tirewear. Those black marks on runways are just a different form of dollar bill.
A .22 rimfire will penetrate any aircraft skin. Any centerfire tracer will create a problem, although the first one through the tank won't necessarily start a fire. The second or third hit will, though...
Spraying from a duster means a highly dispersed "poison". That means a lesser likelihood of infection unless it's some sort of stuff for which only one spore will do the job.
This anthrax deal going around necessitates a large number of spores to be effective.
The people-problem with a crop-duster is that if a residential neighborhood is sprayed, 99% of the folks will be indoors, and the plane is gone by the time anybody realizes what's happened. So, stay indoors for a while. After the spray has had time to settle, go wash your porches, sidewalks and car with water and Clorox. That is, rinse first, and pour Clorox into the runoff.
Art