So to answer your first question. There are basically 3 different types/sizes of AR receivers. The small format is the original AR15. It can be chambered in .22lr, 5.45x39, .223/5.56x45, 6.5 Grendel, 6.8SPC, 300Blackout, 7.62x39. It can also be made (with appropriate parts) to chamber pistol loads like 9mm and 45ACP, as well as large bore loads like .50 Beowolf, .450 Bushmaster and .458 Socom. The large format rifle is often called the 308AR, AR10, and LR308. Those are either misnomers or inappropriate vernacular. The large format was originally intended to shoot .308 and 7.62x51, but may also be chambered in .260 Rem, 6.5 Creedmoor, .243 Win, 7mm-06, and I think I'm missing a few more. There are three basic "styles" that are most common, and areally often most distinguished by the rypeople ofor magainze rhey use. The DPMS variant, the Armalite variant and the third/rarer model that utilizes the HK G3 magainze. There are companies like BOHICA and SERBU who make bolt action uppers for the small and large format lowers. These can be chambered in anything, including .50BMG, .408Cheytac, .416 Barrett. There are even a crossbow and spear-gun uppers for the small format rifles. The last size is newer to the market, and usually VERY proprietary. The Magnum AR can normally be found in .300WinMag and .338 Lapua Magnum. There have been 30-06 and .270 rifles made, but none were successful in their marketing. A 12g and a 20g were attempted on the large format, but the company (Fidelis Arms) failed before the prototype ever got released.