How long is long-term storage? You planning on going cryo or something and waking up in two, three hundred years?
I have some XM193 on stripper clips in bandoliers since the early 1970s. The cartridge brass is mottled green in spots. The ammo still works OK. This is military stuff, "designed by geniuses to be run by idiots" (Thank you, Herman Wouk, "The Caine Mutiny") that has to outlast a nuclear attack and SE Asian jungles. Unless your ammo and barrel are dripping wet with oil or are plugged, chances are it will likely work fine until the next century at least. I shot some WW1 .303 the other week, worked OK, too.
As a guy I met once, an ex-USMC gunnery sgt. w/more obscene (yet funny) tattoos on his arms than you could shake a stick at, who rolled his eyes when I asked him about AR15s and any sort of break-in period with milspec ammo and barrels, said:
"Just shoot the f*cker....."