Tensioned Barrels

[barnbwt]

There are some pretty stout springs on the market, that if you had maybe 5-6 of them surrounding the barrel under the tensioning shroud you could easily put several hundred pounds of pull

I planned on using Belleville washers for this, if I go the spring route. Very little deflection for a lot of force, and they have a truly circular contact area (and the math for them is straightforward). There is an upper limit to how much force I can pull the barrel with, as my tensioning tubes, being aluminum, will eventually crush or buckle much sooner than the barrel will stretch. I'm seriously only looking at a few dozen pounds of force, which is why the disappearing tension is such a problem; there's no way the amount of tensioning force will equal the amount of expansion in the barrel due to heating.

If the tensioning struts expand FASTER than the barrel, the tension will DECREASE faster.

How about HEATING the barrel and then tightening the tension nut?

I think you have that backwards; the struts expand faster, causing them to 'grow' and pull the barrel harder. The problem of course being, again, a change in variables as the thing heats, which makes it harder to stay consistent. My thinking is that simply having *some* tension at the muzzle is enough to get acceptable performance (which seems to be the case for tensioned barrels that I understand shoot great and suffer an abrupt accuracy loss once expansion overcomes their tension).

The problem with the thermal tensioning you describe, is that it would work *too well* and crush the tension tubes. A 26" steel barrel of ~1in2 cross section will expand ~.026" when heated 150 degrees; to rigidly contain that expansion once cooled would result in over 23,000lbs of tension (much of which would be relieved by the tension struts themselves compressing and contracting thermally along with the barrel, but not nearly enough for 2024 aluminum tubes to keep from being crushed). I'm not going for a barrel that meaty, but I think the tension developed would still be in the thousands of pounds . That's why I think some form of 'give' needs to be present in the system, regardless what I do, so those crazy loads can be kept in the 'mere' hundreds of pounds if not less. The root problem is that the barrel is long (i.e. I doubt an 8" Dan Wesson develops these kinds of issues over its short length)

The more I think about it, the thermal compensation idea has the additional complication of heat transfer rate. I can calculate the tension strut cross section needed to maintain non-destructive tension on the barrel as it heats for a slow or steady state condition, but since the struts will heat more slowly than the barrel, again we have the situation where tension drops, only faster for rapid than measured fire. Kinda defeats the point of semi-auto if you lose your accuracy shooting more than two rounds a minute

I think the simple, stiff spring solution is the way to go, since that's the way the math is easiest (and I've found that this means it's generally the best engineering solution). So the barrel heats, the spring tension relaxes somewhat but not enough for the muzzle to become uncontrolled, and after firing when the tension rods heat up, the tension builds up again until the whole thing cools back to ambient. Seems reasonable. Knowing how much deflection to expect from the barrel & rods, and knowing their material strengths, I can calculate how much flex at the washer is needed & how much force it must contribute (and from those last two I can calculate the size/number of Belleville washers needed)

Now the next trick is devising interfaces that allow this movement without galling the hell out of everything, or shifting the gas block hole out of alignment. I'm thinking two conical steel surfaces mating behind the muzzle device, the washers stacked between the movable conical surface and the gas block, which has a slip-fit over the barrel? I don't think the gas ports would ever significantly misalign unless I threw ice water on only the rods or barrel

TCB

 

[rodregier]

FWIW, Dan Wesson revolvers had a tension system for their interchangeable barrels. They made claims of enhanced accuracy.

 

[barnbwt]

Their claims were largely real, too, though the effects are really not realized over something as short as a pistol (especially if limited to iron sights on such a short radius). Mainly, it made it easy to swap out barrels, that was the real purpose.

TCB