It’s not though. The difference in wall thickness is still causing uneven hoop stress forces being exerted on the bullet. If you are using Lapua brass (fairly consistent neck thickness), try just using a free floated bushing with a diameter .002-.004” smaller than your loaded round neck diameter (measured with tube mics and not calipers if you can help it).I'd hafta know *ALL* they are doing. Likely they are neck turning. I am not. So I use mandrels. Quick and easy path to consistent neck tension.
Its like guys that use 108 octane to go fast. I like to go fast, but there's a reason I don't use 108 octane in my car.
Dave