If you have a Dubro igniter, most of the newer sub c cells won't fit in it. A guy just gave me an old (but never used) Sanyo 3600 cell that fit like a glove after I pealed a layer of clear shrink wrap off of it.
Remember most of these cells come with at least 2 layers of shrink so if you have a tight one you can carefully slice one layer off.
Sean. Certified speed crazed mowron.
Team Shepherd USA