Muzzle deflection

[Apollo]

Does anyone know when muzzle deflection basically stops mattering? I've been playing with Lija's barrel stiffness calculator and it is really kind of cool, but I can't seem to find any baseline for muzzle deflection. I mean, I can take the top benchrest competitor's barrel length and profile to find out what kind of deflection they are getting (ran the numbers and it runs about .002" assuming a 1 pound weight was hung off the barrel).

But at what point does it stop mattering? .001" .0001" ? I've read somewhere that tac ops recommends a deflection of .0012 max with most of their barrels being .001

If I ran a barrel with a max varmint contour and a 21" barrel in .338 it would have a stiffness of .001 almost exactly, or half what most top 50 bench rest competitors are running. Anyone have any data or thoughts on this?

[RobertMW]

Hey Apollo,

I never answered you in 707 since I was away from a computer and I figured it was going to be a long explanation. Lets see where it goes.

I think that calculation is relatively pointless. The number that it is going to give you will be a static deflection. What really matters is what happens once the system is dynamic. Really, what you need to do is get your masters in mechanical engineering and focus your studies on Euler-Bernoulli/lagrange bending. Even as a mechanical engineer myself I only know the basics of it and plug stuff into a computer to get my answers, anyway...

vibrations in a material travel at the speed of sound of that material. So when the impulse of firing the bullet hits the barrel the energy of the impulse is going to travel much faster than the bullet, this will be setting up vibrations all along the barrel in different harmonics before the bullet ever leaves the barrel.

The problem with a static deflection is that I could create infinite different profiles that give you the exact same static deflection in height at the end of the barrel, but every one will have a different set of harmonic frequencies and act differently when fired out of.

When you constrain yourself to some fairly basic barrel profiles: straight, tapers, multi-section tapers; you do eliminate a lot of that variation, so I would be willing to bet that most of the best benchrest barrels are going to fall within a certain range of that static deflection. If you just look at the one variable, static deflection, you might make an incorrect assumption that just creating a static deflection in that range will give you a good barrel.

What really happens is that you have to match your barrel profile to your barrel material to your internal ballistics.

What many top benchrest competitors are doing to get such tight groups is not be creating the stiffest system in the world, they are doing it by getting a system that they can minimize the shot to shot variations as much as possible, thus letting their bullet be released at the exact same point of dynamic deflection each time. That point has to be a desired one; any random release, even if you hit it every single shot, will not mean you will have an accurate system.

Your .001" stiffness may mean that you will have a very stiff barrel, even dynamically, which will probably mean that you will have at least a marginally accurate barrel, but you may not have a desirable harmonic set anywhere that is reasonable for you to shoot at. Unless you are getting the 21" MV barrel on recommendation that someone out there has already created a similar system that is super accurate, I think that barrel deflection value is only going to give you a small piece of a larger picture.

As a last disclaimer, you know that I do not have extensive practical experience like some people that may answer on this, but I do have the theoretical knowledge to send you down the right path.

If anyone has practical experience that refutes this, by all means I love to learn more.

[Apollo]

The way I see it, a stiff barrel is easier to tune and find an accurate handload for. That's all I'm really expecting.

[LynnJr]

Apollo
Back before I switched to tuners on most of my barrels very thick barrels were all the rage. To remove the affects you need a very thick barrel on the order of 4 inches.
Check that diameter by looking at Harold Vaughn's book Rifle Accuracy Facts.

I wanted a very stiff barrel but 4 inches was out of the question so I bought a 3 inch blank for my 6BR. The barrel weighed 57 pounds and the gunsmiths with lathes big enough to do the job didn't want to do it.
I asked Dave Tooley and his response was on Benchrest Central but in a nutshell no way.

When you tune your load for best accuracy you want the barrels muzzle climbing. If the muzzle is falling the gun won't shoot.
If the muzzle is climbing slower shots take longer to reach the muzzle so they are released at a higher departure angle than faster shots.
With a tuner attached to your muzzle you can Compensate for a given amount of velocity variation in your ammo and the slower and faster rounds will converge at the target.
It is called Positive Compensation by Dr Geofrey Kolbey.
Another good source is Varmint Al's website on tuners.
He modelled my world record setting 6 Dasher rifle if you can find it. He is a wealth of information and he lives about 45 minutes from us.

[Apollo]

Cool!

Sent from my XT1045 using Tapatalk

[LynnJr]

http://varmintal.com/atune.htm

[RobertMW]

http://varmintal.com/atune.htm

I've always loved the barrel harmonics .gifs from him.

[LynnJr]

RobertMW
Put a 8-12 ounce weight 2.75 inches in front of your muzzle that is adjustable and watch your groups shrink.
They work they are cheap and when set correctly you don't need to adjust them ever again.