More of what Jim Ahrens taught and more can be found in two of my books: Weaving for Beginners and Weaving & Drafting Your Own Cloth.
Matching the bobbin to the shuttle is important. The cavity in the shuttle where the spindle is mounted has either squared-off corners or oval, rounded corners. You need to fit the bobbin to the cavity in your shuttle or the thread will jerk or jam as you are weaving when the bobbin hits the corners. Squared-off corners of the cavity are for bobbins with flanges at the ends—similar to those on the ends of spools sewing thread. This photo shows a shuttle with a squared-off cavity and the bobbin suited for it.
In a round-cornered cavity, use bobbins with extensions sticking out from the flanges. This prevents the bobbin from jamming when it hits the rounded corner during weaving. Bobbins with extensions are readily available and can be used in either type of shuttle.
You can put a small bead or a sewing machine bobbin on the spindle at each end of the bobbin if your bobbins don’t have extensions, and your shuttle has rounded corners in the cavity.