I had something cross my mind an hour or two ago and looked but found no answers. While shopping for a lower (already went with a Stag), I noticed that most lowers now list a specific caliber in the product description. Why is this? I was under the impression that one of the beautiful things an AR-15 can do is change calibers on the fly by switching out the upper/mag. Am I mistaken, or is this some form of marketing for uninformed people who are always emailing/calling support to ask what caliber a lower is?