Calvin,
Not crazy at all. Fact is, I am the only person I know who has tested an action to destruction, just to see how much one would take. They'll take a lot!
Dennis, where would I expect it to fail? Well, My guess is, if you were able to remove the twinsert (bushing, whatever you want to call it), you'll probably find it has cracked in the areas where it is thinned between threads. My guess would be that if a thread or two fail, (cylinder part separates) load will pass to others behind it. At that point, I'd guess you're nearing where it will begin to strip. The good thing about a barrel is that it's got way more threads than it needs. So, even if it were to begin to strip, it'd probably act like a real long, fine thread arrangement, where it'll strip say the first inch of threads, but the next two inches are still intact.
All I can do is speculate. Predicting where something like this could fail is difficult cause it's likely it will do so in a progression. With two different pitches on the ID/OD, means SOME of the threads are going to be basically full depth (at certain spots). Failure at one spot could lead to additional strength at another.
Charles,
I agree, and yet I wonder if a full failure isn't possible. I don't know. I don't think it will blow up, but If someone said one did, I wouldn't doubt them either.
Some time back a discussion began about some bolt lugs and bolts breaking. I commented in that thread about how these things happen, and what to expect when they do. I said this after having made bolts and broken them. Even with the bolt head shattered, it still didn't come out. Nothing breached, nothing escaped... Stuck another one in and did it again. Same breakage pattern. Same results. At least in my actions, I felt confident that with a proper bolt, there was zero chance of anything dangerous, regardless of how bad a bonehead move I could make at the loading bench. I could fill a 300Wby case with bullseye and still not be scared (tho I would not be opening the bolt any time soon!).
I know this, if you purchase a twinsert (got boxes of em at work), there is a considerable amount of material that's contiguous in them. Unlike a helicoil. The reason they need that when a helicoil does not, is that the helicoil is the same pitch inside and out. Each thread is a dual-bearing surface. When they are not the same pitch, they cut each other off occasionally. At those places, the part is thinnest. That is where I would expect it to separate when placed under load, like barrel torque, or firing. Even if it doesn't break there immediately, it will almost certainly fatigue. Still doesn't mean it'll fail, but, after a period of time you might have two pieces of bushing holding the barrel instead of one.
I'd be curious to know how it all turns out.
When I threaded the internal of the bushing and I decided it was threaded enough, I then machined a relief with the threading tool left at that depth... this measured .945. That is how I established that dimension.
If that measures 945, then the thread itself is really smaller than .945 (on the tenon). There is a radius on the tool, so maybe a good bit smaller. Giving a pitch diameter of the tenon would tell more about this. Not that the exact number means anything. It's just a number. The only thing you need to know is how to replicate it for the next barrel. So the internal number is meaningless.
I don't want to turn it into a hornets nest, but it seemed like an interesting discussion.
I know, I pull the trigger on Sako extractored guns all the time, and safety never crosses my mind. But this, now here I might get a ants'y about pullin the trigger here. I could see me doing a string and hiding behind a tree!
