Please help me with an AR upper build

[jwr]

I am new to the 300 whisper, but am interested in building a SBR in an AR 15 platform. I am wanting to build the most compact weapon that I can. Ballistically and functionally what is the shortest barrel that I should reasonably go with. This weapon will mostly be for concealment/protection, but I may want to pop a whitetail or two. I am not so much interested in the subsonic capabilities of the cartridge as I am in the cartridges better terminal ballistics from a shorter barrel than the 223 cartridge. Thanks in advance for your help, jwr.