I got the idea from some of the old timers here, they used a business card the same way, but paper doesn't last well once it gets wet, but the frig magnet is kinda rubbery and should be forgiving to wet weather, so I tried it and it works well on a lot of my H&Rs, cheap and easy to install, and can be removed if it doesn't work, kinda like the O-ring!!

The only issue I've had with it is it tends to stick to the barrel, a little gun grease or oil seems to fix that issue tho. I use a dab of RTV silicone to stick it to the forend so it stays in the same spot when the forend is removed. I can't tell ya how it would do in cold weather tho, I spose I could test it by putting the forend in a cooler before heading to the range and installing it there before shooting tho!!
Tim