The barrel is adjustable for headspace dimension control. This saves a lot of time in the assembly of the rifles. The barrel can be finished all at once, then assembled and out the door. The barrel nut is what holds it all together and allows for fast headspace ajustment. Other companies use the barrel face by the recoil lug to hold it tight but needs to be finish reamed after assembly to get the correct headspace. Funy thing is, there is only a few big name rifle manufactures that have not gone to the barrel nut in one of their rifle lines.