Quick answer: Cracks can begin during drilling, glue-up, turning, pressing or everyday use; diagnosing the stage matters.
Identify when the crack appeared
Write down the stage and stop working the damaged blank. A serious split in a rotating workpiece is a safety concern, not just a cosmetic flaw.
Drill-stage failures
Do not rely on glue to conceal a structural crack in a piece that will spin at speed. Select a new sound blank if there is doubt.
Turning-stage failures
If a tool catches twice at the same place, reassess technique and grain rather than forcing another cut. AAW recommends full face protection and stopping a vibrating machine to investigate.
Assembly-stage failures
If a finished barrel is already split, replacing it is usually safer and more reliable than hiding the line under a coating. A replacement part is a better outcome than giving someone a pen that fails.
Avoid recurrence
Use a blank-selection checklist, record drilling diameter, inspect glue cure, square to the tube and assemble with controlled pressure. Those process measurements reveal far more than changing random accessory brands. If several blanks from the same stock split after finishing, examine moisture condition and material history before continuing.
Failure timeline exercise
Do not keep using a severely cracked rotating blank because its outer finish looks intact.
When replacement is the right answer
A replacement piece is warranted when the split compromises the wall surrounding the brass tube, when a structural defect remains in the rotating wood, or when finished components cannot be safely aligned. Small cosmetic cracks do not automatically mean you can repair the underlying structure for turning.
Sources and further verification
Source links are included to help you confirm equipment specifications, accessories and recommended safety practices. Specifications may change with product revisions.