My wife bought me a T/C Compass in 308 Win for Christmas last year. I have been shooting a 7mm Mag, 6.5x55 Swede, 9.3x62 Mauser and a 375 H&H for decades. Never had anything in 30 Cal so I found myself in a hurt locker for ammo. I have several thousand rounds of handloads for the 4 rifles I mentioned as well as plenty of powder and primers but had nothing for the 308 in the way of brass, dies and bullets and stuff was hard to get. However I did have nearly 500 rounds of once fired 243 Win brass that my late father shot up and I traded that for dies and enough brass/components to get the 308 going.
In my case it wasn't a matter of getting caught short or not being prepared, it was a matter of getting a new rifle going in a caliber I had never owned. If you really want to get ammo and shoot you will find a way to make it happen.
Just the other day, I picked up like 80 , 308 Win Cases.. Mostly Federal.. add that to the zillions I have picked up.
I don't shoot a 308, but it necks down to 260 real easily... and 7/08 to boot.
I've probably picked up at least a 1000, 223 Rem cases in the last month, laying on the ground.. people shoot it them just leave it...