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Tensioned Barrels

Discussion in 'Rifle Country' started by barnbwt, Aug 11, 2016.

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  1. barnbwt

    barnbwt member

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    Anybody familiar with them?

    Specifically, arrangements using compression rods rather than concentric tubes to pull the barrel with, and mechanisms to keep tension roughly constant as the barrel heats (at least for a while).

    I'm not (only) asking because there happens to be a WA2000 for sale on GunBroker (for the low-low price of 65,000 dollars :p), but because I wish to make an inspired bullpup rifle in 6.5 Grendel* with a similar over/under barrel compression beam layout.

    Strangely, there seems to be very little information on barrel tension systems, most I've seen are simple tubes concentric to the barrel that a muzzle nut is torqued against. Commonly reported are stringing issues within two or more shots as the barrel heats, thermally expands, and relieves the tension.

    As stiff as a barrel is, even several dozen pounds of tension will only stretch it a thousandth of an inch or so, several times less than thermal expansion will 'grow' a hot barrel. So I had two ideas, which I have not seen explored in other tensioned barrels online, to bounce off the Collective;

    -Belleville washers or similar stiff springs between the muzzle nut and thrust block (also happens to be the gas block on my design) that allow the barrel to expand further out before tension is completely lost (gradual decline in tension over .003" vs. complete disengagement after .001")

    -Imitate a temperature-compensated clock pendulum, and make the tensioning struts of a material that expands faster than steel (like aluminum) and size them to maintain a fairly constant force on the muzzle across a certain temperature range (will not be effective for rapid fire/rapid heating, relies on stable/consistent temps throughout the materials)

    Or maybe even some combination of the two :cool:. Any rail guns or similar "highly modified" accuracy rifles out there using something like these ideas that met with success?

    TCB

    *Because Kel Tec likely won't do their 6.5 RDB in our lifetimes
     
  2. barnbwt

    barnbwt member

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    Bueller...Bueller...
     
  3. RainDodger

    RainDodger Member

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    Know what a tensioned barrel means to me? Had a Sako once that I couldn't make shoot well until I put a piece of a popcicle stick under the barrel and tightened the receiver screws down on it. Then it shot ok.
     
  4. TikkaShooter

    TikkaShooter Member

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    Get a barrel frozen to align the metal's grain structure.

    RainDodger
    I own an old early 1960s Sako 222R mag, a Tikka 22-250 and a .308 M595 Master Sporter. With quality ammo or reloads, they all are well less than a MOA.
    The 22-250 is guaranteed less than a MOA with specified ammo. It shoots less than a MOA using S&B 55gr. The Master Sporter is a poor man's TRG. Absolutely wicked with Federal BR.
    As accuracy is what you pay for; if I was you, I would have returned it.
     
  5. barnbwt

    barnbwt member

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    I've heard that called a 'shimmed' or 'tuned' barrel, often done in conjunction with muzzle weights or heavy brakes to slow down the muzzle harmonics such that the 'sweet spot' duration where the bullet will exit in line with POA is longer & easier to hit within the narrow band of velocity variance.

    I'm not gunning for a .25MOA/2000yd tracked railgun or anything, looking more to get the barrel profile reduced a bit from "full bull barrel" diameter (it's a ~24" barrel blank, after all) without hurting accuracy, and potentially to stabilize such a long barrel against harmonic effects generally (i.e. consistency load to load, of which there is considerable variation for Grendel). Supposedly tuning weights at the muzzle are just about as effective, but must be adjusted for every type of load used.

    But, mostly I'm just trying to figure out if my schemes of adding "give" at the pressure point are actually helpful in making the taught barrel more consistent as it heats up.

    The premise of the WA2000 was to make a sniper/DMR rifle compact enough to use in an urban environment without breaking light fixtures getting into position, but it was fairly heavy at fifteen-something pounds depending on optic, and the grip/stock/action arrangement really made it a lot bulkier overall than what seems necessary. I also think the 7.5 Swiss cartridge it was chambered in, while awesome, is generally more juice than is needed for such a precision instrument (i.e. it was intended for taking exposed terrorists in a hostage scenario). I figure that since the Grendel has similarish trajectory & accuracy potential as 308 out to at least five hundred yards, it'd be cool to do a half-pint version that weighs a lot less & doesn't cost 3/4ths a Dodge Hellcat while retaining the same basic utility as a 'counter sniper.' Rainbow Six fantasies aside, I personally think such a platform would be just plain enjoyable to shoot from bench, standing, or other positions (especially since this one will eject out the bottom)

    Probably no-go for me, or to be more precise likely not worth the cost; I'll be turning/chambering the barrel from a GM blank & cutting the other parts myself, so the benefits of cryo-treat would probably get lost in the noise of a non-premium barrel blank (to my eyes it looks quite nice for a button-rifled blank to be honest, but it simply isn't a cut-rifled & polished 800$ Bartlein or whathaveyou)

    TCB
     
  6. 4thPointOfContact

    4thPointOfContact Member

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    I've had a tensioned barrel in my M17 bullpup for over a decade and it shoots dang accurately for a gun that wasn't meant for long distance. Having the thinner barrel pulled into tension by the stiffer, aluminum outer receiver was a pretty neat idea by the designers. (That said, there are disadvantages to having a shiny black metal receiver as your cheekrest, but that's easily solved.)
    http://www.kmarms.com/ has a modern interpretation of the M17 but they did away with the barrel tube and angled front plate that allowed the barrel to be tensioned. It could always be added if there are any parts left on any of the numerable parts places.
     
  7. barnbwt

    barnbwt member

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    Did not know the M17s had tensioned barrels, that's pretty cool! So it maintained accuracy after, let's say, two magazines of intermittent shooting pretty well? Very promising, I'll have to look into how they went about it.

    TCB
     
  8. Bart B.

    Bart B. Member

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    “Tensioned” rifle barrels are what made M1 and M14 match grade service rifles shoot so accurate. The best of them shooting good lots of commercial match ammo or good handloads (not reloads) would put 20 shots inside 4 inches at 600 yards; fired once every 15 to 20 seconds. They had 25 to 30 pounds of down pull at the stock fore end on the barrel. You could squeeze the barrel and fore end together and see the gap between the lower band and stock ferrule open up. Bolt action rifles don’t need that. That tensioning helps mitigate the front end of the op rods bouncing around too much inside the gas cylinder as there is a couple thousandths clearance between the rod’s gas piston and the cylinder; just like car engines but with out expanding piston rings to seal the combustion chamber. You don’t want that piston bouncing around too much causing extra vibrations in the barrel as the bullets go through it.

    Why not have your rifle built like match grade bolt guns without any tension on the barrel (they’re totally free floating touching nothing but the receiver) and will put 40 to 50 shots fired several times a minute into 2 inches at 600 yards (yes, that’s been done)? They (along with service rifles) shoot rapid fire matches at 200 and 300 yards with two 10-shot strings each fired in a 1 minute time limit’ they’ll put all ten somewhere inside 2 inches at 300 yards in proper tests. In one special rapid fire match, M1, M14 and M16 rifles are also shot 24 times in 50 seconds at 600 and 500 yards. While nobody shoots them as accurate as a much slower rate of fire would permit, 25 shots inside 8 to 10 inches is not unheard of.

    Those rifle’s barrels are fit to receivers whose face has been squared up with the barrel tenon thread axis. The barrel shoulder fits hard and even all the way around it. Not just at one point where commercial rifles do so a stress line’s made in that direction in the barrel as the metal heats up and expands. Have your receiver face squared up then put a shim between it and the barrel when put back together so the barrel clocks in to correct headspace.

    Shimming the barrel with something between it and the stock fore end guarantees any change in rifle hold or position will transfer through the bending fore end to th barrel. But it does help a poorly bedded receiver shoot a little better. Nobody builds a super accurate bolt action rifle that way. The down pull on M1 and M14 rifle barrels do require than once the fore hand is slung up with a sling and in position to shoot, do not change the elbow's position else bullet impact will change. It's a lot more change than what bolt guns do because of the tight fit of barrel to stock at that point.

    On barrel "tuning" . . . .

    Nobody tunes a rifle barrel such that the 'sweet spot' duration where the bullet will exit in line with point of aim for best accuracy. If they did, the bullet would strike below the point of aim down range; gravity at work. When the bore's parallel to the line of sight, that's when it moving the fastest. And if that were possible, all the bullets exiting on the down swing will have the faster ones go above and slower ones below the point of aim; negative compensation. You want the bullets to exit on the barrel’s up swing so the opposite takes place; positive compensation. And the bullet must exit at a greater angle than the line of sight to compensate for bullet drop down range. To learn about this, check out:

    22LR Rifle & Tuner
    Esten's Rifle & Tuner
    Light Rifle & Tuner
    Barrel Harmonic Movie
    Barrel Tuner Analysis
    6PPC Barrel Dynamics

    in this web site: http://www.varmintal.com/

    You may observe the best place for bullet exit is usually on the up swing just before reaching the high point. Anywhere on the up swing is better than anywhere on the down swing.
     
    Last edited: Aug 13, 2016
  9. silicosys4

    silicosys4 Member

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    There has been something called the "accustrut" for mini 14's available for a while, might be worth looking at. It doesn't tension the barrel but it does limit harmonics and help prevent heat stringing

    http://accu-strut.com/

    ASRM2-BTBC_closeup02.jpg
     
    Last edited: Aug 13, 2016
  10. RainDodger

    RainDodger Member

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    TikkaShooter.... a bit difficult to return a heavy barrel L579 Sako manufactured in '73, which I bought from a private party... unfired and in the box. :) I did get it shooting pretty well though. It's a .243. Even without the shims.
     
  11. rodregier

    rodregier Member

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  12. barnbwt

    barnbwt member

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    At least that link had some decent close-ups, if not a whole lot of technical info (no one does). It certainly appears that the muzzle block attached to the rods is a type of clamp, a block of metal that flexes as bolts squeeze a slot milled through it closed, onto the barrel. That by itself would not tension the barrel, but perhaps it is squeezing down on a conical surface, driving the barrel forward as it clamps down?

    As I said, very little overall deflection is required to tension a barrel, but a whole lot of force, so such an arrangement would have to flex very little to clamp up, and would still maintain a very close mating fit between the block & barrel after doing so. I wish we could know how thermally stable the Walther was (it was semi-auto, but I don't know if it had rapid fire accuracy requirements beyond a handful of shots)

    Unlike Walther, I plan to use this clamping piece as the actual gas block, so if it truly functions as a barrel clamp and tensioner, that actually works well for me (since I'd like to be able to remove the thing for modifications during test easily). Maybe instead of a single 'sprung' block of metal, do it in halves that clamp down onto the compression strut seats/gas tube somewhat like a barrel vise?

    TCB
     

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  13. CB900F

    CB900F Member

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    Barnbwt;

    Once upon a time, a long long time ago, I built my daughter a 10/22. That gun was equipped with a Volquartsen tensionlite barrel. The gun was, and for all I know continues to be, outstandingly accurate. This was before that company began building their own guns. I don't think they offer that exact barrel anymore, but you might consider asking them about their experience with the concept & why they dropped it.

    900F
     
  14. stubbicatt

    stubbicatt Member

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    How about a variation of the muzzle device called the B.O.S.S. which Winchester had on some of their rifles in years past? I've seen them on F Open rifles, and their owners shot very well indeed?
     
  15. rodregier

    rodregier Member

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    The BOSS was for "simple" tuning of resonance for a specific loading.
     
  16. armarsh

    armarsh Member

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    I have to question this $800 figure. The last Bartlein barrel I bought was $325 last year. I purchased two Kreiger barrels last month at $340 ea. All Kreigers are cryogenically treated. (for the same low price :)).

    Yes I understand this is double the cost of a Green Mountain blank. But it seems to me that the goal is to get what extra accuracy is possible for the least amount of weight and cost. My opinion is that the cost of the tensioning system will be higher than the upcharge for a premium barrel and it remains to be seen if it would out-perform the premium barrel. Also, cut rifling barrels have very low internal stresses and tend to move very little with heat.

    If the goal is to do a tensioned barrel simply because it needs to be done :evil: , I understand.
     
  17. Bart B.

    Bart B. Member

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    I'd like to know what the real accuracy of those externally tensioned barrels is. Largest groups shot with a given barrel and the most accurate make and type of ammo.

    I ignore smallest groups for anything. There's no way to determine if they happen when everything is perfect, or all the variables in rifle, ammo and what holds the rifle pretty much cancel each other out in all directions. The largest groups shot show what all the variables can add up to. If you shoot enough few-shot groups with anything, one will be sub 1/6 MOA. The guy who holds the 100-yard 5-shot benchrest record of .0077" holds no other records and all his previous groups were a little to very much larger.

    The best conventional barrels in 22 rimfire rifles will keep 50 shots tested at 50 yards inside 1/4 inch with good match ammo; 3/4 inch at 100 yards. Sometimes with a tuner weight, other times without. Any externally tensioned rimfire barrel ever done that?

    I, and barrel makers I've talked with, don't think a barrel heats up enough to change its resonant frequency and harmonics thereof enough to matter as they heat up and expand. Stainless steels expand about .000006" per inch for every degree F they heat up. If they're completely free floating, they will whip and wiggle at the same resonant and harmonic frequency from ambient to very hot temperatures as normally fired; slow or rapid fire. If they touch anything as they expand, the harmonic frequency of the last few inches of the barrel will change as will it's whip axis. Bullets now take off in some other direction than desired.
     
    Last edited: Aug 14, 2016
  18. barnbwt

    barnbwt member

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    Correct, I'm referring to chambered barrels here, and more generally how expensive a 6.5mm anything is compared to nearly any other diameter. At any rate, even the 100$ GM blank is a bit higher than I'd prefer (was really torn with 6.8SPC since the price is about half for that diameter). Budget has priority for this kind of project, and if it ends up I really like the thing & it's not quite as accurate as I'd want, I can make a second barrel at that point :cool:

    The tensioning system is a 4ft 2024 tube cut in half, about 10$. Your point about internal stresses is certainly a good one; I could probably get away with a much lighter profile barrel using a cut-rifle high quality tube, for no other reason than they warp less when you turn more than 1/3 of the material off in my limited experience (old military barrels vs. button rifled blanks). GM's tend to be...risky to thin down too much since you can end up with a straight exterior & bowed interior bore path that can result in a much thinner wall than planned.

    It's about 80% this. I figure I might as well give a legitimate shot at the tension system, vs. simply adding a purely cosmetic feature :p

    This is one of those "pick 2" scenarios ;). I'm going for low cost prototype, minimal weight/size within the confines of the long barrel & steel/aluminum materials, and trying to maximize accuracy along the way where I can. So long as the crown is cut right and I chamber it correctly, it's gonna be ~2MOA at worst pretty much no matter what I do. I'm thinking the barrel tension is an easy way to halve that for at least the first few shots, but perhaps a mag or so if I can keep it from 'evaporating' as the barrel heats up.

    One other benefit I've seen touted for tensioned barrels; they are "harmonic resistant." Apparently, because the muzzle is better controlled, shooters have found different loads will shoot consistently, obviating the need to carefully test a spectrum of loads to find those that line up with the 'accuracy nodes' inherent to barrel vibrations. Basically, a given precise load will more consistently shoot to a given POA load-to-load vs. a freely vibrating barrel that will "like" some loads much more than others --at least that's the claim. That'd be great for Grendel, seeing as it shoots anything from <100gr varmint pills to >123gr VLD javelins, sighting in new ammo could be easier.

    TCB
     
  19. Vern Humphrey

    Vern Humphrey Member

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    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? That way, as the barrel cools, the tension increases, giving you more tension than you could achieve by merely tightening the tension nut.
     
  20. RecoilRob

    RecoilRob Member

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    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 (if they'd be on the beginning of the barrel I guess the more proper term would be 'push') on the barrel with more than enough wiggle room that the barrel stretching out a few thousandths would't change the tension much if at all. I think.

    Would look a bit weird but a 'ring-O-springs' around the base of the barrel might be doable. At least the components for a test rig might not be prohibitively expensive enough to let you try the idea to see if it's viable. If so...then the MKII version could be tarted up to look nice. I believe the idea of the tensioned barrel is sound...and using that to stiffen and support the barrel proper would free you of some of the wall thickness needed seeing as much of it is to prevent whip and distortion as the bullet travels down it like a snake that ate an egg. Externally holding it might very well allow a nice trim barrel that would be lighter and cool faster. Will be a cool experiment anyhow if you try it!:)
     
  21. rodregier

    rodregier Member

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    Perhaps tension the barrel, apply the clamp and then release the tension.
    (As a "shop" build procedure).

    A version of that is how they make pre-tensioned precast concrete structural supports
    with interior reinforcing.
    (Concrete is strong in compression, poor in tension.)
     
  22. desidog

    desidog Member

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    Ever shot an early skinny barrel Ruger Mini14 or Ranch Rifle? Mine would walk up the page with each shot. Up and to the right.
     
  23. Bart B.

    Bart B. Member

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    Ruger barrels, as well as others not fit to the receiver, all change the amount of whip in one direction as they heat up. The frequencies they whip at don't change.

    Ruger's have had a receiver alignment issue with barrels. Adjustment zeroed scopes mounted in rings on them often end up running out of adjustment to get shooting zeros. The barrel tenon threads are not square with the receiver face and torqued in barrels bear very hard at one point around their shoulders. This condition in any rifle causes shot walking as the barrels heat up.
     
  24. Corn-Picker

    Corn-Picker Member

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    I wanted to say thanks for all of your informative posts. Whenever I see your username I know that expert level information is forthcoming.

    I agree with what you say about looking at the larger groups. Two years ago I had a three shot sub-MOA group at 100 yards, from the standing position. I can never achieve anything close to that consistently, but if you shoot enough groups some of them will be very small.
     
  25. Bart B.

    Bart B. Member

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    Corn-Picker, I've put three consecutive shots inside 1/10th MOA at all ranges up through 1000 yards. Once in a great while. With both scope and metallic sights on the rifle. With .308's (bolt guns and Garands), a .264 Win Mag and a couple of 30 caliber magnums.

    I don't claim they're sub tenth MOA shooting rifles. But everyone's rifles and ammo are that way. If they shoot enough 3-shot groups, they'll finally get one. Some sooner than others.
     
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