I assembled this model a while back, but then it sat in my loft (or attic to my American cousins). With the new kits coming, and all the extra arms, I wanted to make the arms interchangeable for flexibility. The new crusader build in particular looks quite interesting (albeit expensive points wise).
First up I removed the arms. I had assumed, Games Workshop would give spare arms including the shoulder joint (silly me) so I have magnetized them already.
I then cut along the joint between the shoulder and elbow connectors using a fine saw. You need to be careful here as there is not a great amount of space. Then file the surfaces flat.
I then drilled out the the the centers of the these pieces using a 6 mm drill bit until I could fit a 6 mm x 2 mm rare earth magnet. Its really important this hole is centered, and I recommend using a pilot hole first. This was a bit of a pain and I would recommend a narrower thicker magnet in hindsight.
Taking care to get the right polarity, I then glued the magnets in place. I used the first arm (in my case a chainsaw arm as the master. Using a spacer magnet to prevent accidental glue issues, I inserted the magnet into the hole to test the size. If its fine, insert the magnet carefully ensuring it is flush and centered. If not fill and start again.
Voila, a snap on snap off weapon configuration.