M4-Based Short Barreled Rifle Barrel Selection Considerations

I am planning to assemble my first Short Barreled Rifle (SBR) based on the M4 Carbine platform of my own, and I have been trying to do a lot of reading about how to go about. In the course of my research, I have come across various interesting aspects that I would like to share with other newbies out there.

One of the more interesting facts I found out are the factors one has to consider when choosing the barrel length for an SBR. First, just a quick review of the term "Short Barreled Rifle": It is taken from the United States' Bureau of Alcohol, Tobacco and Firearms (BATF) definition of any shoulder-fired, rifled firearm with a length of less than 16".

Also, note that the standard M16/M4 barrel comes in four general classifications. Below are a listing of them, with the maximum length for each classification noted down next to it:
- Rifle Length, 20"
- Mid-Length, 16"
- Carbine Length, 14.5"
- Pistol Length, 7.5"