AR-15 Upper Receivers come in many variations. There are A1, A2, A3, M4 and many different barrel lengths and profiles. One of the most common is the M4 style which has a barrel length of just over 16″ with a notch on the barrel for a grenade launcher and features a flat top receiver. The flat top receiver allows you to add many accessories quickly. You can attach flip up sights, fixed sights and any number of optics that are available.
The A1 sports a fixed carry handle and while it does not readily accept some optics, there are mounts that make the attachment of optics possible.
The A2 is very similar to the A1 in that it is a fixed carry handle, but it adds a much improved rear sight assembly.
Varmint uppers are popular for many hunters due to it’s lack of a front sight post and the flat top upper. It provides a clean sight picture when you mount a scope to it.
The sky is the limit when it comes to caliber. Now, there are uppers available in calibers ranging from 22 lr to 50 BMG. Most caliber conversions only require a different barrel or upper and different magazines. The larger calibers may require a different lower as well.
An upper in 22lr can be an excellent choice if you like to shoot a lot. You can shoot hundreds of round of 22 compared to a few of the 5.56 rounds.
If you like hunting, an upper in 6.8 may also be a good choice. The 6.8 utilizes the standard 5.56 lower, so just swap your upper and mags and you are ready to go deer hunting.