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Barrel bending

Started by dgiannandrea, December 03, 2021, 05:41:47 PM

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dgiannandrea

My springer shoots way too low. I added a drooper mount, shimmed the rear ring, dialed huge amount of reticle, made my scopes complain, shooting horribly. Finally realized the the reticle wasn't stable at the far end of adjustments.

Then, I searched how to bend a barrel and went to work.

I used a carpenters level as true. Examined 4 sides of the barrel, taped drawings indicating problems and simply used a padded wood vise while leaning into the problem. Nothing fancy, just padding, weight and muscle. I examined opposite sides of the barrel was a way of measuring twice, bending once.

The I centered the reticles, mounted the scope directly (no drooper mount, no shims) and started the final tune: Shoot, bend, repeat. I'm now letting the barrel rest after bending POI withing 1" of crosshairs at 15 yards.

A huge amount of thanks to all who have documented this and provided me with the process and confidence. I would suggest that this should be part of the setup process for springers and and essential to save your scope.

Thanks all.

steveoh

Quote from: dgiannandrea on December 03, 2021, 05:41:47 PM
My springer shoots way too low. I added a drooper mount, shimmed the rear ring, dialed huge amount of reticle, made my scopes complain, shooting horribly. Finally realized the the reticle wasn't stable at the far end of adjustments.

Then, I searched how to bend a barrel and went to work.

I used a carpenters level as true. Examined 4 sides of the barrel, taped drawings indicating problems and simply used a padded wood vise while leaning into the problem. Nothing fancy, just padding, weight and muscle. I examined opposite sides of the barrel was a way of measuring twice, bending once.

The I centered the reticles, mounted the scope directly (no drooper mount, no shims) and started  the final tune: Shoot, bend, repeat. I'm now letting the barrel rest after bending POI withing 1" of crosshairs at 15 yards.

A huge amount of thanks to all who have documented this and provided me with the process and confidence  . I would suggest that this should be part of the setup process for springers and and essential to save your scope.

Thanks all.

You are a brave man! I've read about folks doing this and it made me feel just a tiny bit woozy thinking about it. :o
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Alan

One thing to keep in mind. Once you do this, the barrel will droop again, but a bit sooner than before. And each succeeding time, the condition will get worse. The only way around this, would cost more than the gun is worth by several times.
Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

nervoustrigger

I can't fathom yielding a steel barrel
by way of cocking it, even a beast of a springer.  Once you've "un-drooped"'a barrel by bending it (yielding it), that is as permanent a fix as there is.

Alan

I am not a metallurgist, but I know what happens eventually. Every single thing, when stressed enough times, will fail. The barrel of a airgun is no different. In fact, straightening it adds another layer of stress.
Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

nervoustrigger

#5
Well I'm not a metallurgist either but fortunately the properties we are talking about here aren't terribly complex.

Bending the barrel to address the droop was achieved by exceeding the material's yield strength.  Meaning, that's the point at which the steel goes from an elastic (recoverable) bend to a plastic (non-recoverable) bend.

The property you are referring to is fatigue, where a material is stressed over and over again.  A material's fatigue limit defines the point at which you can stress it an infinite number of times without causing it to yield and fail.

For steel, these two values tend to be pretty close to each other.  So for example, mild steel:

250 MPa - yield strength
200 MPa - fatigue limit

Notice the fatigue value is 80% that of the yield.  Meaning however difficult it was to bend the barrel permanently to fix the droop, you'd have to apply almost that much to it every time you cock it to ever fatigue it.  Not to mention it would take hundreds of thousands or perhaps millions of cycles.

That's why we can consider it a permanent fix in almost any conceivable situation.

Alan

I guess we could beat this horse a bit more, however, I do have a question. How did the barrel get bent in the first place? I'd also like to know, what the bending forces on the fulcrum are? This obviously would require a bit of measurement of the levers involved. But is has to be many times that of the force applied to the barrel.

Aside from all of this, the weak point isn't the barrel. Rather it is the hinge. A few break barrels have dovetail fittings, to assure the barrel is properly seated. Seemingly, it doesn't matter, as they're still difficult to shoot accurately. Just as bad, is the double recoil most of them have. I had a UMAREX Octane, in .177. It went through three scopes, before I gave it away!

Obviously, my experience with break barrels hasn't been good. They are noisy, and finding a pellet which will shoot in one, is a task all in itself. Ne'er again for me!
Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

dgiannandrea

When I started the process, the barrel had an obvious bow in it, not entirely in the down direction. First I took care of the bow, got the barrel true measuring left, right, top, bottom. That took care of the middle.

Then I started shooting and tried to correct close to the breach.

I was surprised to find so much light between my straight edge and barrel when I started. I figured if the manufacturer didn't care that much about straight why should I. In the end it's not about how straight the barrel is, it's more about the last harmonic at exit. Straight and true reduces friction, increases fps. Where the last inches of the barrel points is where the pellet goes. And I admit a bent barrel introduces another force on the pellet but I don't think it's greater than that twanging spring.

The rifle is 20 year old Diana 48, .177, side lever, used to me. I don't think it was abused as the stock is in great condition. The previous owner replace the spring with a 12ft-lb version to control POI and thereafter claimed great accuracy. So I suspect this thing came that way from the factory and at least 3 owners have suffered adjustments/compensations to make a well respected rifle perform as it should. My ability to test the limits on this gun is lacking, but I'm a lot happier now with how it shoots.


Gerard

If you're not concerned about the looks, an option to 'set' the corrected bend might be to bond a slightly over-sized CF sleeve over the whole thing. You would have to notch in the part of the carbon tube where it has to pass over the cocking linkage, but it should be possible with some careful Dremel work to get a tube seated nicely with just a tiny bit of play between steel and ID of the tube. Then clean the steel well and apply a coat of JB Weld steel filled epoxy to both the barrel and the inside of the tube and slide it into place, blocking muzzle and breech of course. Wipe down any squeeze-out with an alcohol soaked paper towel, as anything outside the tube isn't relevant for stiffening the barrel.

I've stiffened several airgun barrels this way, and have made a .22lr barrel liner into a barrel by soldering it into a steel block at the breech end, with a recessed step of 1/2" for a 2mm wall CF tube to slide into about 2" ahead of the breech face. The result is a surprisingly accurate barrel on a bolt action rifle, weighing much less than a full steel barrel but much, much stiffer after leaving the epoxy to set for a week. I've similarly added stiffness too a .22" LW barrel I put on my QB78D, and that thing is consistently sub-MOA with JSB 15.89gr at 850fps. I don't see why a CF tube well bonded to a break-barrel wouldn't help resisting barrel droop, while providing protection against future bending via cocking strain.

Motorhead

Quote from: nervoustrigger on December 08, 2021, 06:01:22 PM
I can't fathom yielding a steel barrel
by way of cocking it, even a beast of a springer.  Once you've "un-drooped"'a barrel by bending it (yielding it), that is as permanent a fix as there is.

Would agree ....
Having bend compensated MANY barrels for droop, the effort to get barrel to actually bend requires WAY way way more than it takes to cock it via the barrel.

Non issue ....
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