Their are extensive lists online that specify the dozens of cartridges that are offered for each size lower.
The reality is that there are really only a few cartridges that are readily available for the public to purchase straight off the ammo shelves at a retail store. Most of the rest are specialty rounds or handloaded.
While you may have one lower, you could have 5.56, 6.8SPC, 6.5 Grendel, and .300BO for uppers you'd like to use on it.
Then the next complication rises - different cartridges have different purposes. They are specifically designed to be different in what they do, not universally interchangeable. One round may be meant for long distance accuracy, another for increased power in combat, the third for subsonic use in short barreled suppressed uppers.
That kind of different use then goes to the lowers and the different stocks, grips, and controls someone might prefer for those more specialized applications. If the market shows us anything, the majority of shooters don't prefer a one size fits all stock.
The long distance shooters tend to prefer a "fixed" length stock that accomodates prone position shooting, the "combat" or competition shooter a light weight adjustable carbine, and the suppressed SBR shooter a short fixed entry team stock for it's battering ram abilities.
So, can you pin "any" upper to a lower, sure. But most don't, it's an internet fantasy. Uppers grow lowers and the gun rack gets another finished gun. Search the internet for multi caliber owners posting gun rack photos and racks with just "uppers" are few and far between. We talk about it but we don't do it.
