I make it a habit to buy ammo at the same time as a new gun and I buy about 250 to 500 rounds and then set it by knowing I will be ok for a while without causing a shortage with future purchases....LOL. I was lucky I bought 1000 rounds of Lake City 2020 50BMG right after I got the rifle and the gun came with 10# of powder and 750 rounds of brass. And when I got my self protection side arm, a P40, I won 400 rounds of S&W 40 just a few days later in a contest. I have 2,000 rounds of 7.62x39 and God only knows how many rounds of 9mm Para Bellum that I bought at 19 cents a round when I got my P08 Luger.
If I buy the ammo when I get the gun I dont need to worry about the ups and downs of the market.
My recent purchase of a KelTec KSG-25 shotgun that will hold 41 rounds of shorty 12GA required some looking around for the ammo I wanted, 12GA, 1 3/4", slugs, but I did find it, finally, the same way I found the gun, with an ongoing search on Ammoseek.com. Got 200 rounds for 25 cents a round. I will never shoot that much unless an army attacks the house. BUT the gun does hold 20% of my stock at one loading so I figured 200 rounds was not too much.