Nice idea on the magnet,.. do you modify a specific shirt to hold the other half or just a small plate of metal in a pocket of your chosen shirt/jacket ?
Them NEO magnets are strong I used some once in an IPSC match
to hold loaded mags to my pockets since the stage didn't let you load from your pouches the mag kind of "floated" on my side as I ran to the second shooting station then reloaded... the RO wasn't sure it was legal but they let it slide..
so this year I am putting neo magnets on my O/U to magnetize the shot as it goes down the barrel it should tighten up the pattern cause of the magnetism holding the shot together longer as it exits
Seriously though a cool use of magnets
John