The spiraloc thread system works. I used to do fatigue testing of fasteners for the Auto industry. That being said I cannot bring myself to threading a barrel shank in that manner. You can make your own tool by putting a small angle on the tip of your normal cutting tool. Spiraloc will sell you the inserts, just tell them your a gunsmith.
We used to build cannon breeches at a company I worked for there were a few in our lobby. The threads on those were square or modified trapezoid as I recall. They were a centralizing thread fit meaning the flats on the od and id aligned the barrel. Square threads were used by a lot of those old actions etc. garand, springfield etc. Someone fixed the newer actions to use a v thread. I always felt some flat or coned areas on the tennon would be and improvement over just the v thread. I think the European actions incorporate this technology. Grunig and others.
We used to build cannon breeches at a company I worked for there were a few in our lobby. The threads on those were square or modified trapezoid as I recall. They were a centralizing thread fit meaning the flats on the od and id aligned the barrel. Square threads were used by a lot of those old actions etc. garand, springfield etc. Someone fixed the newer actions to use a v thread. I always felt some flat or coned areas on the tennon would be and improvement over just the v thread. I think the European actions incorporate this technology. Grunig and others.