I figure that if I’m going to have to carry significant amounts of ammo and it’s a SHTF scenario (house is unsafe to stay in, food shortages, desperate people, mob behavior, etc.) then I’m loaded with a heavy hikers backpack, as is my wife, and 4 little ones each have a small backpack with what they can carry (small mylar bags of water, some food, clothes, etc.). The packs my wife and I carry will have the balance of the heavy stuff including ham radios, batteries, lighting, water filtration, mres, food bar rations, tent, blankets, fire, cooking, fuel, etc.
With my pack being a good 30lbs, and with my <7lb AR on a 2 point or 1 point sling on front, I think I’d fill my 4 mags with ammo, then somehow pack another 200 rounds at least somewhere…burying the other 1500 or so rounds left behind. I’d also have a holstered glock 21sf and 2 additional mags on my belt for that, and maybe bring another 100 rounds of that somewhere.
We’d hit the railroad tracks and head out of town stealthily as possible. Or float out on the Bitterroot river if we could arrange it. RR tracks or river because the roads would likely be compromised and not low profile. Also, I’m assuming gas is long gone and our cars are useless. We would head to less populated mountain valleys facing south, or to near enough cities.
Short answer, having said all that: 320 rounds of 5.56 and 139 rounds of .45acp. Also, none of us are in shape enough to hump around 1000 rounds of anything, and I doubt carrying our packs would even be possible, but we’d try.