Thank you for this advice and experiences. I think this does make sense that there could be internal stresses and a little tap might allow things to return to a stress relieved state.
Please elaborate. I use a very thin o-ring (0.8mm CS) in this manner, but only to add a small amount of friction to the barrel/moderator interface to preclude the moderator from loosening with extended shooting sessions. This approach seems to work well, as the o-ring basically resides mostly in the small ID entry chamfer of the moderator threads when the moderator is tightened down. It is still "metal-to-metal" at the joint, but I only have to "snug" it (not super-tight) due to the added friction the o-ring provides. It has not become loose unless you want to remove it.
The reason being? It's worked very well for me (for years now) & others I know. If you think it's bad advice, don't take it.
Threads are effectively a circular ramp whose purpose is to exact mechanical advantage to provide a clamp. In order for this to happen there must be a running clearance between the male and female parts to prevent seizing. Threads cannot provide concentric alignment because of the required running clearance. Concentric alignment, when required, is always provided by a pair of mating reference surfaces, never the threads. The issue we have with mounting a moderator is that it is happen chance. If you look at any other concentric alignment requirement, like on machine tools, it is always provided by a pair of very clean mating machine surfaces never threads alone. By introducing a compressible material, you have introduced another alignment variable. There can be a multitude of reasons why when tightening the mod, it introduces a skew like the binding surface out of alignment with the barrel or just a small piece of debris. Whatever causes a skew cannot be corrected without first detecting it. In that light, a straight test rod machined to exactly fit the land diameter of the barrel for at least 2 inches and extending through the exit of the moderator is a quick easy test. The rod diameter can be smaller than the MOD bore. Your eyeball can easily detect if the rod does not exit in the center of the MOD bore. I very often remove and re-install my moderators without any issues and they never loosen, but I have experienced misalignment, This is where the test rod is invaluable.
Steve--very good explanation regarding achieving concentric alignment. I think THAT occurs only when we have two mating surfaces that are perfectly flat and "square" (true?) with each other. If these mating surfaces AND the threads are well-cut (as in performed on a lathe, with parts "dialed-in" using an indicator, this is pretty much a given. The threads, in this case, will also help self-center mating parts (you mention "circular ramp" in your post, I agree) because of the "ramping" effect of the threads between the two. The tiny o-ring I mentioned serves only to provide a small amount of additional friction, such that it prevents inadvertent loosening of the moderator. Remember, the o-ring resides only in the chamfered entry area of of the moderator, and does NOT intrude (extrude?) between the two flat mating surfaces. With only minimal compression, the o-ring should have no effect on concentricity. Your "check rod" suggestion is a good one, and really gives one a visual check on concentric alignment of the moderator. Also, in lieu of a check rod, a jewelers loupe can also help examine the moderator exit, and should easily reveal any "clipping" of pellets is that is an issue.
Yes, you are partially correct, but as an example most FX guns use an adapter that fastens to the shroud, not the gun barrel. The new HUBEN GK1 (both v1 and V3) uses an adapter that screws onto barrel threads and the adapter first clamps the shroud to the receiver. The adapter threads will assume a skew if the mating shroud surface is not square to the barrel line. Further confusing the issue is that flat mating surface of the adapter that the moderator should clamp against is slightly lower than the face of the shroud. That allows the moderator to clamp against the shroud not the adaptor face. These are just some of the examples that I am aware of. I am sure there must be even more. All of which can and in many cases do, induce a moderator skew.
Right? The challenge for me was not knowing as much as I now know. The gun and scope is 27" x 12.5" x 4" and I was struggling to find a case that didn't seem so excessively long. Moderators weren't even on the radar at that point... thought they were prohibited in Massachusetts. Wisdom is like experience, something you don't get until just after you need it. Clearly joining this forum first would have been so much smarter. Still don't know what case I would get if I started over. Plano or pelican... then pick foam or cut it out. I think going to some shooting competitions and seeing what is actually used will be helpful.You're putting together a sweet airgun rig that will easily cost over $2,000 once you factor in scope, mod, etc. Why would you build it around a $50-100 case?? Know what I'm sayin?
