Well....... I have learned a lot as a result of researching this little problem.
I consulted with an expert in Europe whose whole life revolves around threaded joints - albeit not with Rifles. He didn't do an analysis (and probably wouldn't without being paid), but was willing to offer the view that the sloppiness of the threaded joint would rule. A loose fit would allow the thread misfit to be accommodated but tightening torque should be reduced to avoid metal yielding since fewer threads (than normal) would be taking the load. With a tight fit, yielding of the threads at both ends would take place due to jamming before proper clamping force could be reached. A good compromise would be very hard to achieve. The bottom line - he didn't recommend doing it.
I also reviewed chapter 6 in Harold Vaughn's book in detail. While others here didn't see any benefit from doing a ramp thread, Harold seemed to like it because it did show positive results for him. However, he stopped just shy of recommending it for everyone. Harold's analysis of the number of threads holding the torque load was also interesting. However, I found it a little hard to swallow the distorted plastic model since plastic has nowhere near the yield strength of steel and would significantly magnify and concentrate any effect. Perhaps he used plastic to help his readers understand his point. Nonetheless, it is a well known fact that proper clamping torque varies with the length of a threaded connection all for the very reasons that Harold described. Shorter connections demand reduced torque and threaded connections longer than necessary result in no additional advantage with the bolt end probably "flapping in the wind" as one poster suggested.
In the end I decided to do my own 3d-Cad model of the joint and discovered a big nasty. IF (A VERY BIG IF) the thread mismatch were reversed, with the barrel being metric and the receiver being SAE, I believe it might actually result in a superior joint that would accomplish everything that Harold was trying to achieve through his research. I'm actually surprised that Harold didn't think of it! Theoretically, one could deliberately mismatch the thread pitch to achieve an optimum result IF one could get their hands on a lathe with an infinitely variable thread pitch (way beyond my meager means). That's because the interference would build from the end of the tenon toward the shoulder compensating for the tension expansion of the joint as it is tightened. I know that is a very simplistic explanation, but for those who understand my point, its good enough.
Alas, that is not the situation here. In our case, the pitch mismatch between the barrel tenon and receiver is reversed which results in even fewer threads at the shoulder end of the barrel and receiver taking all the load than normal, and the bolt end being even looser than normal. Tightening the connection to standard torque levels will no doubt yield the first few threads until enough following threads are presented to absorb the axial torque load. Lower torques can be used to avoid this, but accuracy will no doubt suffer to some extent for all the reasons that Harold outlines so well.
I might also do some work on a junk rifle to test double threading sometime in the future. Basically put two short sections of 16tpi thread on the tenon - one near the shoulder and one near the bolt face with nothing in between, and with the two sections of threads offset by slightly more than the barrel tightening torque would stretch the steel. (Yet another approach to Harold's problem.) But that's an experiment for another day. I mention it here only to generate thoughts and ideas from others. Obviously, this approach might also prove useful even in rifles with standard SAE threads in both the receiver and the barrel. Anyone who wants to steal the idea is welcome to it! Just let me know how well it worked - good or bad.
The bottom line for me right now is simple. a 16tpi barrel in a 1.6mm receiver is probably just fine for a hunting varmint rifle, but I am not going to do it on a competition rifle. Its too hard to get the joint looseness just right, and the risk of noncompetitive inconsistent shot-to-shot movement of the joint is just too high for my liking. So the hunt is on for some custom end gears for my lathe that can do 1.6mm threads. If I can't make that work, then I'll return the rifle to my customer and tell him that I can't and won't do the work for him for all the reasons discussed above.
Others here say that they have had better experience having actually done the work and having achieved satisfactory results. The proof is on the paper so to speak. My hat is off to them. But quite frankly, given what I have learned here with the help of others, I now have very very little confidence that I could be so fortunate at the level of performance that I am seeking.
Thanks everyone. Your frank views and experience have been very informative and are much appreciated.
I consulted with an expert in Europe whose whole life revolves around threaded joints - albeit not with Rifles. He didn't do an analysis (and probably wouldn't without being paid), but was willing to offer the view that the sloppiness of the threaded joint would rule. A loose fit would allow the thread misfit to be accommodated but tightening torque should be reduced to avoid metal yielding since fewer threads (than normal) would be taking the load. With a tight fit, yielding of the threads at both ends would take place due to jamming before proper clamping force could be reached. A good compromise would be very hard to achieve. The bottom line - he didn't recommend doing it.
I also reviewed chapter 6 in Harold Vaughn's book in detail. While others here didn't see any benefit from doing a ramp thread, Harold seemed to like it because it did show positive results for him. However, he stopped just shy of recommending it for everyone. Harold's analysis of the number of threads holding the torque load was also interesting. However, I found it a little hard to swallow the distorted plastic model since plastic has nowhere near the yield strength of steel and would significantly magnify and concentrate any effect. Perhaps he used plastic to help his readers understand his point. Nonetheless, it is a well known fact that proper clamping torque varies with the length of a threaded connection all for the very reasons that Harold described. Shorter connections demand reduced torque and threaded connections longer than necessary result in no additional advantage with the bolt end probably "flapping in the wind" as one poster suggested.
In the end I decided to do my own 3d-Cad model of the joint and discovered a big nasty. IF (A VERY BIG IF) the thread mismatch were reversed, with the barrel being metric and the receiver being SAE, I believe it might actually result in a superior joint that would accomplish everything that Harold was trying to achieve through his research. I'm actually surprised that Harold didn't think of it! Theoretically, one could deliberately mismatch the thread pitch to achieve an optimum result IF one could get their hands on a lathe with an infinitely variable thread pitch (way beyond my meager means). That's because the interference would build from the end of the tenon toward the shoulder compensating for the tension expansion of the joint as it is tightened. I know that is a very simplistic explanation, but for those who understand my point, its good enough.
Alas, that is not the situation here. In our case, the pitch mismatch between the barrel tenon and receiver is reversed which results in even fewer threads at the shoulder end of the barrel and receiver taking all the load than normal, and the bolt end being even looser than normal. Tightening the connection to standard torque levels will no doubt yield the first few threads until enough following threads are presented to absorb the axial torque load. Lower torques can be used to avoid this, but accuracy will no doubt suffer to some extent for all the reasons that Harold outlines so well.
I might also do some work on a junk rifle to test double threading sometime in the future. Basically put two short sections of 16tpi thread on the tenon - one near the shoulder and one near the bolt face with nothing in between, and with the two sections of threads offset by slightly more than the barrel tightening torque would stretch the steel. (Yet another approach to Harold's problem.) But that's an experiment for another day. I mention it here only to generate thoughts and ideas from others. Obviously, this approach might also prove useful even in rifles with standard SAE threads in both the receiver and the barrel. Anyone who wants to steal the idea is welcome to it! Just let me know how well it worked - good or bad.
The bottom line for me right now is simple. a 16tpi barrel in a 1.6mm receiver is probably just fine for a hunting varmint rifle, but I am not going to do it on a competition rifle. Its too hard to get the joint looseness just right, and the risk of noncompetitive inconsistent shot-to-shot movement of the joint is just too high for my liking. So the hunt is on for some custom end gears for my lathe that can do 1.6mm threads. If I can't make that work, then I'll return the rifle to my customer and tell him that I can't and won't do the work for him for all the reasons discussed above.
Others here say that they have had better experience having actually done the work and having achieved satisfactory results. The proof is on the paper so to speak. My hat is off to them. But quite frankly, given what I have learned here with the help of others, I now have very very little confidence that I could be so fortunate at the level of performance that I am seeking.
Thanks everyone. Your frank views and experience have been very informative and are much appreciated.
