[elkaholic1]

I read somewhere, possably on this forum, that it is unsafe to flute a factory barrel. The OP seemed to have a pretty logical answer, but I can't find the post again. Anyone know the facts on this? If it's true, I wonder why so many 'smiths offer the service?

[Alpacca 45]

Why would you?

Vaughn (rifle accuracy facts) has produced a good and believable argument for weight, not stiffness being important in damping barrel vibrations.

People will offer to do all sorts of things if they can get payed for doing it. whether it is worth doing or not, or is even beneficial to do it, doesn't come into it.

As to whether its safe, that depends on a lot of things, caliber and chambering, thickness of existing steel and steel spec, you can get a rough idea from assuming a 50% above SAAMI max pressure chamber and bore pressure 50,000 PSI yield stress for the steel and plug the figures into the formula for an internally pressurised thick wall cylinder the diameter of the bottom of the flutes. if the stress in the bore exceeds the 50,000 psi, then make the flutes shallower and try again.

[elkaholic1]

Purely for looks, somewhat to cool the bbl faster.

So as long as too much pressure is not put on the barrel, it's generally safe?

[rcmodel]

Yes it's safe, if you start out with a heavy barrel.

It would probably not be safe on a light sporter weight barrel.

The problem as I see it is not safety, but that removing metal from a barrel by milling flutes in it sets up internal stresses that were not there before fluting.

Unless the barrel is stress relieved after fluting, accuracy may suffer.

rc

[krs]

This is from the FAQ's at Shilen Barrels www.shilen.com :

Q)What about "fluting" a barrel?

A) "Fluting is a service we neither offer nor recommend. If you have a Shilen barrel fluted, the warranty is void. Fluting a barrel can induce unrecoverable stresses that will encourage warping when heated and can also swell the bore dimensions, causing loose spots in the bore. A solid (un-fluted) barrel is more rigid than a fluted barrel of equal diameter. A fluted barrel is more rigid than a solid barrel of equal weight. All rifle barrels flex when fired. Accuracy requires that they simply flex the same and return the same each time they are fired, hence the requirement for a pillar bedded action and free floating barrel. The unrecoverable stresses that fluting can induce will cause the barrel to flex differently or not return from the flexing without cooling down a major amount. This is usually longer than a shooter has to wait for the next shot. The claim of the flutes helping to wick heat away faster is true, but the benefit of the flutes is not recognizable in this regard until the barrel is already too hot".


I do know that this was written before the original owner of Shilen Barrels passed away, and think it's notable that the new owners have left it on the website, apparently subscribing to the technology and continuing the restriction against fluting a Shilen barrel for the reasons stated.

Take it as you wish.

[Jim K]

Read and heed the Shilen statement.

As to how you know when the fluting is too deep? Easy. When the barrel splits and maybe takes the receiver ring with it, doing damage to a few fingers or eyes for good measure. Then you know not to cut so deep the next time.

Jim

[elkaholic1]

How would one "unstress" a bbl? Cryo treatment?

I dont plan to cut flutes myself, I would leave that up to a pro...if I did it at all. I ask the question as it stems from a friendly discussion on the merits of fluted varmit bbls. Shilen makes a good point,two bbls of the same weight the fluted one would be more rigid as it would have a larger OD.

What I'm curious about is...I don't see how cuting flutes would be more stressful on a barrel then cutting the rifling, provided you did it before the bbl was unstressed.

[Alpacca 45]

Assuming that you have a stress relieved cut rifled barrel, the rifling cuts were made using an ultra sharp single cutter, taking perhaps a few hundredths of a thousandth of an inch per pass, and taking one cut in each groove of the barrel for each advance of the cutting edge. Total depth of each finished groove is perhaps 4 thousanths of an inch.

Fluting cuts are made using a multi tooth milling cutter. This is run so that each tooth starts to cut at the base of the cut and it produces a chip which starts at a few tenths of a thou or more thick when the tooth begins to bite, and gets thicker as the tooth rises up towards the top of the cut. (running the feed the other way (climb milling) risks the cutter snatching and bending the barrel and breaking the cutter)

Before the tooth begins to cut, it will skid a little way, smearing the surface of the steel, leaving locked in stresses.

Because the cutter has multiple teeth, it is almost impossible to get each one identical height and cutting identically, so some will skid further and some will dig in harder. Milling is always a series of shock loads, no matter how good the cutter, and the full depth of the flute may be made in one two or maybe three passes, rather than the hundreds it took to cut a 4 thou deep rifling groove.

I see cryo treatment touted, and the best justification I've seen for it is "it increases the life of cutting tools so it must be good for barrels too"

There is a big credibility gap there. In heat treating steel, it is cooled rapidly from above it's "Critical temperature". the critical temperature is the temperature above which the stable phase is called "Austenite" which is a none magnetic solid solution of carbon in iron and the alloying elements.

During quenching, hard iron carbides are precipetated (e.g. "Martensite") however in some steels, the reaction is slow and some austenite may remain, frozen outside its stable temperature range. Cryo treatment takes that austenite even further outside its stable temperature range, forcing more of it to convert to hard carbides.

In cutting tools and measuring tools like guage blocks which are meant to be accurate to fractions of a wavelength of light (not just tenths of a thou like rifle barrels), there may be a treatment of a series of cycles of heating and cooling, intended to force any austenite to invert to martensite and ferrite, which are both stable at normal temperatures.

For normal barrel steels, soaking for 24 hours at around 400 C is generally enough to allow machining stresses to do their bending and stabilise the barrel.

If the barrel was button rifled or hammer forged, there may be even more interesting stress effects. Add to that that the stresses induced by these cold forming processes are often beneficial in countering the stresses caused by firing the gun.

You'll find much better discussion of stresses in steels if you go to the library (local town or local college or university) and check out some "strength of materials" text books.

Getting back to vaughn and "Rifle accuracy facts" he found that the ideal for damping down the wildest vibrations is a less stiff and heavier barrel, ie the opposite of what fluting is doing.

from a deformation rather than a strength point of view, you want to keep the bore and any flutes at least 3 bore diameters or flute diameters (which ever is bigger) away from each other. I'm not saying that will avoid deformation, but get them closer and you will get significant deformation.

hope this makes things clearer

[Alpacca 45]

As I posted earlier, lots of people will do things if you pay them to. Whether the thing is good to do, or worth having done doesn't come into it:

You get what you'll pay for
and
You'll pay for what you get

they are not the same thing

[elkaholic1]

Alpacca 45 -Wow, great explaination. Thanks for taking the time to put that all down. That give me plenty of sound info to win the arguement :D

What is Vaughn's first name? Is the book called "accuracy rifle facts"? I'm always looking for more reading material to expand my knowledge base.

[Alpacca 45]

I think it was harold, his book is published by PS magazine
http://www.precisionshooting.com/books.html

It really lays a lot of myths and old wives tales to rest

[Bart B.]

Barrel fluting; one of the most "myth and misconception" filled subjects on the internet as well as real talk and print media.

Check this link for some interesting and factual info:

http://www.snipercountry.com/Articles/RealBenefitsBarrelFluting.asp

Alpacca45's comments are worth remembering. If one's ever watched a Sheffield air gage being run up and down a finished barrel that's been fluted, they'll drop their jaw twice; once at the bump in diameters at the back end of the flutes and again at the front. They'll also notice the groove diameter under the flutes is different than at the unfluted section; larger for button rifled barrels, smaller for hammer forged ones. If that's how you want your rifle barrel's bore to be, go ahead and flute it.