300 Blackout vs 223 : Which is the best AR15 round

I will be purchasing a .300 Blackout AR-15 upper soon. The cartridge is getting very popular and I can find it on Walmart shelves. I decided on this rather than a semi auto AK-47 rifle because there are more AR-15 rifles (over 8 million) in the United States and because it uses standard AR-15 magazines which are $13 or lower in price presently. Your assessment is about right on the range of 7.62 x 35. The .300 Blackout beats the 5.56 in power out to about 250 yards if you are firing it using 110 to 125 grain bullets supersonic. Beyond about 275 yards is extreme range for this cartridge. I don't believe it is very effective beyond about 275 yards because of the bullet drop and lack of velocity but 5.56 is not very effective at these ranges either with its lack of energy.

My second cartridge of choice for my AR-15 is the 9 millimeter parabellum. It very cheap to shoot and train with and a much better cartridge for self defense over the .22 Long Rifle.