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For standard vehicle weapons you want something in the region of 1-2mm thick, up to about 5mm in diameter at the most. Alternatively, you can go for the deeper, thinner magnets if you have plenty of room to drill into the weapon/sponson; something like a 4-5mm deep, 2mm in diameter.

For infantry, something like a 1-2mm thick, 2-3mm diameter magnet will be fine for smaller weapon swaps but you may want several, or a beefier magnet for larger metal weapons/backpacks/etc.

At 2x3mm, these magnets will probably work reasonably for both purposes: http://elementgames.co.uk/paints-hobby-and-scenery/magnets/rare-earth-magnets-3mm-x-2mm-x1-]]> http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554234.page http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554234.page GMT http://www.kjmagnetics.com/

The size though, I'm not sure of, I usually buy a variety because they are cheap.]]> http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554262.page http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554262.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554314.page http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554314.page GMT SM arms and hands 1/16" x 1/32"

For larger stuff 1/8" x 1/16"]]> http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554455.page http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554455.page GMT http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554502.page http://www.dakkadakka.com/dakkaforum/posts/preList/685570/8554502.page GMT