the basics are to -
1, find a supplier of rare earth magnets as they are the strongest,
2, find a drill bit which the same size (
eg for my
fw dreads i found that the lug hole in the arm is about 4mm so bought 4mm magnets and hand-drilled the body to fit in a magnet here too),
3, get your polarity right! a magnet has a north pole and a south pole - north attracts to south & same poles repel - i suggest you use a marker pen to mark the end you want showing,
4, glue in the magnet with superglue - maybe add some green stuff the make sure it fits 100%,
5, wait for everything to dry before seeing if you've done it right!
your assumption re the weapon options is correct. however for some awkard sized models (
eg some cavalry) it can also be useful to make packing away the model between games safer & easier.
here's an
eg of one of my dreads: