I just measured mine and it is right at .25 inches in diameter and 9 inches long. They put a small o-ring on each
end to keep it more centered in the magazine tube.
Been using 1/4 inch PLASTIC coat hanger pieces in 2 500A moseys.
light and does not collect moisture. couple shakes and it drops out mag hole. worked since 1976.
I have used both the 1/4" dowel and the plastic coat hanger, and found that the "couple shakes and it drops out mag hole"
is why those O-rings are there, to keep it inside.
I used to carry my gun to the field with the barrel and action separate so I could use a smaller take down case.
One day I found myself in the field reloading after a flight of doves and found I could fill my magazine. Sure enough, when we got back to the truck, there was the dowel on the ground where I had put my gun together.