That pin hole was probably caused by fuming red nitric acid produced as the gun powder deteriorated. Gun powder is breaking down from the day it leaves the factory. Based on an old paper I looked at “Long Term Compatibility Testing of Double Based Propellants”, McCarty 1974, Temperature accelerates the breakdown, powder will totally deteriorate in hours if at 250 F or higher, days at temperatures above 175 F, weeks to months at temperatures around 140 F, years at temperatures above 100F. If temperatures are kept to 70 F or less the lifetime should be in decades.
Gunpowder is tested by heating in ovens at 150 F. If the stuff fumes in 30 days or less it is unsafe for continued storage.
When gunpowder breaks down it released NOx and nitric acid is a by product. Nitric acid gas will attack brass producing cracks, and what I see in your picture, pinholes.
These images are from a 1969 and 1970 Safety Report. You can see that the military was dumping WW2 ammunition forty years ago. The gunpowder in WW2 ammunition has not gotten better since them.