There are a number of ways to measure it — unit measures per round or per SKU, or financial measures — and most vendors keep sales figures close-hold, but one major online ammo seller, LuckyGunner.com, has posted some significant statistical information. It’s deep inside a post that leads with information on website visits, but this link takes you right to the product information.
There’s also what appears to be a quirk in LuckyGunner’s accounting in that they treat the functionally interchangeable (at least in mil-spec weapons) .223 and 5.56 as separate units. Combined, they outsold the runner-up 9mm as a percent of revenue. (If measured in units, .22LR would probably dwarf the others, but rimfire ammo is so cheap that measured in revenue it’s a small thing).
The top 10 individual Stock Keeping Units for product revenue include five 5.56/.223 SKUs, four 9mm SKUs, and a single .40 caliber SKU for 1000-round cases. Those 10 SKUs accounted for roughly a quarter of LuckyGunner’s revenue.