The rearer the caliber, the harder it is to find brass. Not many are shooting it, there is little money in making it. Even common 30 cal magnums there is difficultly in getting brass and ammo.
At Sportsmans there were no magnums in the thousands of rds of ammo in stock, but a 2 of boxes of 300 WM and about 4 boxes of 6.5 PRC...no other magnums were present.
If ya want to find ammo in stock, it's 223, 308, 9mm. Of course a few other non magnum calibers are sparsely represented.
Stock up if you find your favorite magnum brass. Starline is reportedly, 6 yrs behind in brass orders.