But, it could also be a build-up of lead at the end of the chamber that the longer case fits into then expands to fit the lead lump profile. In that case it just needs someone with an Outer's Foul Out or some Wipe Out No-Lead to get in there and get it truly clean.
Can you get a clear close-up picture of a fired and extracted long case? Scuff marks should show whether you have erosion damage or a fouling accumulation. So will calipers. Measure the diameter of the fired case from the mouth down half its length in very small steps. If length near the mouth is is wider than the middle, you have the erosion problem, but if it's narrower, you have fouling accumulation.
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