You can take a piece of 304(usually not> ferromagnetic) and beat the
crap out of it with a hammer and it'll
> become > ferromagnetic because you've changed the structure

You are absolutely correct!  Not only can cold working (beating the crap
out of it) make 300 series magnetic, but welding can do the same.  The
magnetic test is therefore not perfect!  However, the amount of magnetism
is slight however compared to 400 series.

> There isn't really any such > thing > as "crap" stainless, but only
stainless that is "crap" in certain
> uses... > i

Agree - while I would refer to 302 or 304 as the "good" stainless,
because of my exposure to only 410 / 430 and 302  / 304 at my work, where
my wife worked they considered 302 "crap" because it would corrode in
their application where 316 would last!   Even "variations" affect
corrosion resistant.  302HQ (heading quality) is used for most fasteners,
but it is not as corrosion resistant as "regular" 302.

John