I make my vents for my Golf ball mortars and larger all .177 diameter. This size will fit the quill and most friction primers as well as the 1/8 inch standard cannon fuse. Most cannon fuse sold is 1/8 inch.
Some where I saw a rule of thumb for making the vents, equaling 1 1/2 times diameter of the fuse. The reason given was so the that you could be sure that the fuse could be fully inserted inthe charge. That should not be an issue if you properly worm the vent
For years the only fuse we could get was the 1/8 inch green cannon fuse. It was used with most model cannons sold and with out any concerns reports of lost of pressure.
When you get into the smaller cannons under 45 caliber then you might have some concern about pressure lose with the bigger fuse. The smaller firecracker cannons for example would better served with the smaller pyrotechnic fuse of 1/16 inch.
A good source for cannon fuse is
Cannonfuse.com. They are located in Montana and give excellent service--same day or next day shipp via U.s. Mail. They also have in the past supported our cannon shoot here Montana.
Before you drill your vent determine where you are going to get your fuse its size and drill for that size fuse. It won't do you any good to drill a small vent hole for a small fuse and all you can get is larger.
Some thing else to keep in mind is the burn rate of the fuse. Standard green cannon fuse burns at the rate of 30 seconds a foot. There is faster fuse. You will need a longer fuse with fast fuse to get your self clear before the cannon fire.
Lately I have been using FFFFg intstead of fuses in the SAMCC gun with good results.
