Which two ranks can the collar emblems not be switched?

The collar emblems of different ranks in the US Marine Corps are designed to be distinct and represent specific levels of leadership and responsibility. In the case of the ranks of Gunner and Colonel, these two ranks are from vastly different strata within the military hierarchy. A Gunner is a technical expert, often focused on specific tasks and duties related to their occupational field, while a Colonel is a senior officer responsible for significant leadership roles and oversight. The differences in these ranks' roles and responsibilities make it clear that their insignia should not be interchanged, as doing so would misrepresent the authority and capabilities associated with each position.

The other rank combinations mentioned have more closely related roles or are within the same general rank hierarchy, making their insignia potentially more interchangeable or similar, although it is generally not done. However, the contrast in hierarchy and responsibility between Gunner and Colonel underscores the reason why their emblems cannot be switched.