Ok, 300yds is not 'long range', so this gets way way easier than you're making it here.
Just focus on serious accuracy within this application, as this trumps all else..
Assuming you reload(wasted thread if not):
You can go standard 12tw, or 14tw, and use flat base BR bullets ~52gr.
And with this, you can load develop to achieve clean cold barrel accuracy
I don't have an AR15 anymore, but if it were me for your use, I'd set it up with a ACOG type (both eyes) sight, Harris swivel bipod, best cut rifled barrel, and best trigger available. And I wouldn't deploy the gun until I could could kill a 2" dot at 200 -one shot, on demand.
With components available today, I think this is a reasonable goal. But I don't think you'll get there using spears just between point first and blowing up.. These bullets just don't shine up close.