http://yarchive.net/gun/platings.html
There are two common types of nickel, electroless nickel and nickel plate
now often called "electro nickel". Electro nickel is cheaper because the
bath doesn't wear out - you just add more nickel. It is usually put over
a layer of copper to increase adhesion. It has a coarse grain structure
and tends to build up at the high current density areas of the part, giving
an uneven plating thickness. It is somewhat softer than electroless but still
hard. The nickel itself is quite hard but if it is thin, the hardness you're
sensing may be that of the underlying copper.
Electroless is more expensive but gives finer grain structure. You can
put up to 4 or 5 mils of electroless nickel down. It has a hardness of
Rc 45-55 as laid down but afer a bake-out at 350-400 F for 3-4 hours, the
hardness goes up to Rc 60-65.
Nickel plated guns have nickel over copper. I believe these are subject to
attack from the usual chemical cleaners such as Hoppe's #9 and harsher
because the nickel is thin enough or has enough microgaps in it to allow the
chemicals to attack the underlying copper plate. But electroless and hard
chrome guns shouldn't be attacked. That's the theory, anyway. Me personally,
I use Breakfree on plated guns because I _know_ it can't hurt plating. (And
I'm a big chicken.)
JHBercovitz@lbl.gov (John Bercovitz)