(If your a Juggalo, you may need to skip this due to the concept of magnets.)
I wanted my marines to have the ability to switch out weapons and use all those left over bits., so I went a little magnet happy over the years and moved on to using it on other armies. I have found some fun tricks that work and others that haven't.
ALWAYS pay attention to the polarity and what way you are gluing them in, if you do it wrong it will be obvious. Make sure you glue all the items in the same way to make them interchangeable.
Plastic and resin are your best bet, I have managed to use metal to some success. Weight of the pieces can be an issue and sometimes weapon will droop. I had this issue on a metal termi and even a plastic carnifex.
I found that drilling out the hole is better then just gluing the magnet in place, it provides a snug fit and wont need as much glue. You can practice on thick sprue pieces with your drill, find the right depth and mark your drill bit if needed. Marine arms are good to start with, you can cover up mistakes with the shoulder pads you apply later. If the hole is not lined up correctly, you can usually glue the shoulder pad into a position to cover it correctly if the arm is too low or if you manage to drill through the entire arm. Generally aim for the center and go slow watching the depth so you don't go too far. Being too far is better then too shallow, you can glue the magnet flush and it wont be a big deal.
Here we see the 1/8 drill bit lined up with the magnet. If you drill it smooth enough it will be a snug fit so you wont need much glue to hold it in place.
The easiest way I found is to take a stack of magnets and glue the first piece in, you slide the stack off so the glued one wont try and follow.
In this lower picture you can see the sizes of magnets I have used. The largest on the bottom are way too powerful and I have since stopped using them. From switching out large items like dread arms all the way down to infantry arms or even heads, there is not much you cant magnetize. If you can find contact points and get a drill in there, you can get it done. You can see a metal greyknight arm on the left, it seems to work pretty well for being metal.
for the small ones I use magcraft disc magnets, they come in packages of 100 sizes are 0.125 diam 0.063 thickness part number nsn0566 with a 1/8 drill bit.
Sizes and use can very, the larger the magnet the more strength it will have to screw up the glue job.