As Semisane said, it's doubtful that you have a problem. If your exterior flats are still flat, then you either have a small tooling flaw or again like Semisane stated, some interior metal loss due to corrosion. That is actually an area that is common for that. If you have an auto parts store close by that rents/loans tools, see if they have a small bore micrometer set. My set was made for rifles and goes all the way down to 5mm. It goes up to 6 inches deep so you could get measurements before and after the larger section for comparison. The pin heads may or may not be small enough to check groove depth on one from an auto parts place but you could still check bore.