I've assembled dozens of rods, all with shrink wrap grips, never had any slip. The ID of the wrap needs to be within the parameters of the rod being wrapped, so that 50% shrinkage will make for a tight hold on the blank. If the ID of the wrap is significantly larger than the diameter of the largest section to be wrapped, that's gonna be a problem. Use the right amount of heat, keep it moving on the wrap, start at one end (generally, the largest rod diameter) and move along so that no air gets trapped 'tween wrap 'n' rod.