My son-in-law was sighting in a new rifle this weekend. He was using factory ammo - Federal "Fusion". He had already sighted it in with other ammo, and was checking this batch for consistency. Groups were plenty good, but on the case of the third shot, there was physical evidence of burned powder residue around the entire case neck all the way back to the shoulder, but not on the shoulder.

No evidence of case stretching, either vertical or in diameter on that or any other case. No bulges. No visible cracks. No "shiny" rings. I micro metered all the fired cases and they are all well within specifications listed in the Speer Manual for 308 Win. No other evidence case deformation or any sort of pressure problems. I have never seen this before.

Any ideas ?

The rifle is a Ruger Model 77 Mk - II, "Lightweight".