All of the accuracy potential in an AR-15 resides in the upper assembly, other than the trigger, there is nothing in the lower receiver that has any bearing on the rifle's accuracy potential.

If the barrel is correctly installed and the upper is properly assembled, it should shoot.

Is it free floated? Besides a good barrel, this is the most important accuracy upgrade on an AR-15.

Is your gas tube properly aligned? If its torsioning against the barrel nut or receiver keyway, it will adversely affect accruracy. Ideally, you want the gas tube to have the least amount of contact possible with the notch in the barrel nut and the keyway. It needs to also be as close to perfectly aligned with the gas key on the bolet carrier as well- you don't want it dragging.

Check the barrel crown.

I'm not sure what you tried for loads, but it should be very easy to find a really accurate load with 69gr SMKs, Winchester brass, CCI 400 or Winchester WSR primers and 24.5-25.5 gr of Varget or RL-15. If it won't shoot with one of these loads, something is wrong with the rifle.

BTW, my first was a Bushmaster too, it had a few problems that I eventually got sorted out.