"empty thread" groove, on male thread, is

essential to getting the can in line with the bore. Threads don't hold anything straight. All they do is hold parts TOGETHER. There has to be a "shoulder", larger in OD than the male threads, which the rear of the female-threaded part can "butt up against". The empty thread is a groove machined 360 degrees around the male thread, right at the base of the shoulder. Without this "relief groove", the female and male parts will not face up against each other, 360 degrees around, and thus they will not be straight in line with each other.

This is not to say that the tube can't be bent, or the threads can't be crooked, either the female or male or both, in their respective parts. Those issues have to be dealt-with, too, or the can won't be mounted in line with the barrel.