In small gun you don't. Big gun uses 1/4 guns on a long delay refire timer to simulate them. I've heard of some big gun groups using 1/4 inch rods for torps, but I don't know how true that is.
Go to: http://www.bderc.com/ Follow the "Products" link, then "Cannons / Cannon Plans". Order the "Weapons Manual #1" ($20 currently), which shows how to build 1/4" ball bearing torpedo guns. JM
I know that's been tried, but I don't think anybody is using them anymore. As I understand, the problem was that the rods would stick in the hole, keeping it plugged, until the ship scraped past something (like another ship, etc.) that would catch the rod & rip a big gash in the side. One case, not enough damage; other case, too much. JM