I generally agree with both usages. While 500 packs or 10 boxes of 50 have been called "bricks" for many, many years, ammo makers like Federal market 550rd "bulk packs" or "value packs" with the incentive to the customer that you're getting a "brick" with 50 free rounds.

In my experience, I see the term brick commonly used in reference to 500 or 550 packs interchangeably. Any of the other quantities are referred to as something else (like the goofy 350 or 325 round packs meant to fool casual buyers into thinking prices havent changed much becuse this packaging costs as much as 500 packs used to cost 8 years ago.)

IC