Mine has a ground mum but it still shoots great. I had an older model rifle with the mum intact once. A 6.5MM type. The outsides of the gun were super excellent condition but the bore was a sewer pipe, unsalvageable in any sense. So maybe that corresponds to the legend I heard about them peeing in the barrels of guns so that when captured they would be rusted up?
Remember the military is the countries largest organized rumor mill.