There is another kind of "surplus" ammo, not at or near the end of its shelf life. This is sometimes actually ammo purchased by the military, and then excessed (sold).
More often it is ammo made for the military, but then not bought by them. (contract overruns). While not technically military surplus ammo, it is often sold as such.
A bud of mine worked in Calibration and Gaging and he went TDY to a number of US Army Ammunition plants. These plants are Government Owned but Contractor Operated (GOCO). If the contractor finishes the Government Order, they are free to operate the plant for themselves. So you will see LC or similar ammunition, to all appearances military ammunition, it is made on the same machines by the same people to the same specifications, and so it is indistinguishable from military issue ammunition except for the packaging.
This stuff is good stuff, a bud of mine bought 5 gallon bullets of primed 223 LC brass from Midway. It is however new, not removed from inventory because it was old.
Bad milsurp ammo doesn't have to be old WWII era stuff, I had a batch of Israeli stuff from the 80s given to me to pull down. A friend bought 500 rnds (sight unseen) and when he opened the box the stuff was dirty (as in actual dirt) and we test fired 100 rnds. 11 failure to fire (ever) and 17 split cases (ruptures at the case head). Pulled down the rest, the bullets are good, the rest was trash. Deteriorated powder was the cause. Possibly due to excess heat during its stored lifetime.
It could be due to excess heat or just age. The lifetime of gunpowder is unpredictable or indeterminate. Indeterminate means unknown, not predictable, not infinite. Air Force personnel use this term and get confused between indeterminate and infinite, they are not the same. We know that heat and exposure to ions, or humidity, accelerates the deterioration of gunpowder, but your bud's gunpowder could have gone bad without any help.
Look, surplus ammunition is cheap for a reason, it is beyond its shelf life. Some times the stuff will blow up firearms because of burn rate instability, a fancy set of words that means it no longer burns in a predictable way. Don't pay top dollar for surplus, don't expect it to last, and stop shooting the stuff if you get pressure indications.
