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Posted: 9/14/2004 4:02:10 PM EDT
I am about to build AR, I am going to order a kit from model1sales. What are the pros and cons of having the barrel futed. thanks
Link Posted: 9/14/2004 6:17:33 PM EDT
[Last Edit: 9/14/2004 6:18:35 PM EDT by brewsky101]
It's lighter, more surface area so the bbl will cool off faster. Some people say it stiffens the bbl, but I don't know how REMOVING material from a bbl could stiffen it.
Link Posted: 9/14/2004 8:04:08 PM EDT
If you have a barrel and you flute it, the whole surface area = cooling, plus whatever other aspects is true. That said, unless you're firing in full auto shouldn't be that big of a deal.

This is the way the "stiffening" aspect of fluting was explained to me and the more I think about it, the more I think the person was right.
If you have two identical barrels and then send one off to a gunsmith to have fluted, the non-fluted barrel will be stiffer (hence more accurate).
If you have one barrel fluted and one barrel not fluted and both weigh the same (meaning the barrel that is fluted, before it had the flutes cut out, would be heavier than the currently non-fluted barrel), then the one with the fluting done would be stiffer. Basically, the geometry of the fluting makes a barrel stiffer in proportion to its weight but it does NOT make a previously non-fluted barrel stiffer.

So...if you want to save a bit of weight or fire full auto, then it's something to consider. If you want accuracy, then don't flute. Hopefully that explains it well enough. If not, reply back and I can give an example.
Link Posted: 9/14/2004 8:16:15 PM EDT
Tests of fluted vs. non-fluted have been done here before and the results on cooling are that there is little or no difference.
If you are building a LW carbine I can see saving the ounces with the flutes otherwise, I would spend my money on better optics.
Link Posted: 9/14/2004 8:21:34 PM EDT
[Last Edit: 9/14/2004 8:38:19 PM EDT by Boomholzer]
No enhancement in accuracy, probably a unnoticable degregation in precision. No barrel is stiffened by adding flutes.

All else equal, and given the same material, one needs to add mass to a barrel to improve rigity. I think all will agree that a heavier "bull" barrel will provide greater precision over a standard tapered "sporter" barrel.

You cannot remove material from a barrel and also increase it's inherent rigity. A common misbelief. Bushmaster once (maybe they still do) made this claim on their website stateing that purchasing the fluting option to a heavy profile AR barrel would increase it's rigidity.
NOW, one can find clever ways of reducing mass and while offering greater rigidity when compared to a barrel of equal mass, and that is what fluting achieves.

It is very difficult to give a precision decrease in MOA due to adding flutes to a target barrel. I would not argue that there isn't any degregation whatsoever. However, other inconsistancies and errors which take place from shot to shot surely are greater than the detriment from adding flutes. A .5MOA decrease in precision due to properly implimented flutes is absurd. In a autoloader it is unnoticeable, and many F-class bolt-gun guys have no problem with them.

There are other more complicated mechanisms in play which are attenuated or atleast controlled with barrel mass, namely barrel harmonics. This is a area where I would be most concerned with the addition of flutes. Discontinuities along the barrel can cause reflections of barrel harmonics creating different kinds of vibrations (frequency) and intensities (amplitude) along the length of the barrel. In a AR with a gas block hanging on the barrel, this point is silly. I believe that a more gradual taper into the flute trough is better than a steep descent.

I would ignore any benefit of "increased cooling". From a thermodynamic standpoint, the added surface area and depth of the flutes is too minimal to add significant (if any) increase in radiative and conductive heat transfer. Given the three wide and shallow flutes on my LTR, I would confidently argue that it actually would cool slower than it's non-fluted counterpart. Many narrower and deeper flutes will have a greater effect.

Flutes should always be be positioned a equal radian distance from each other around the circumferance of the barrel and be of equal length. This cancels uneven expansion when the thinner parts of the barrel's thickness heats unevenly to the thicker parts. In other words, the flute trough will heat faster then the adjacents crests. Equality is the key. "Slabsided" fluting is a bad idea, ideally the forces should still cancel out. However there are greater unequal tensions in the barrel when comparing top and bottom to the sides. The lesser the number of flutes and the greater the though width and depth, the greater the difference in barrel tensions during heating.

Without significant increases in cooling, a fluted barrel will have a higher increase in temperature per shot than a non-fluted barrel. Given the same energy and efficency in each shot, a lesser mass will heat up faster than a greater mass. A greater temperature rise per shot introduces more deviation in the barrel's characteristics due to temperature.

My .02: I would never flute a barrel on a weapon that I solely intended for the bench for I think that one is adding more inconsistancy although I admit they are very small. If you want to balance the weapon, weight the stock.
Wanting a tack-driver than was managable to shoulder & carry, I think adding flutes to a heavy target barrel is a viable solution.
I think the appeal to most people is that it looks cool. Fluting a HBAR profile AR barrel seems unnecessary and silly.

You are ONLY actually gaining something if you get a heavy target profile LARGER than say a HBAR and flute it to return it to the HBAR's profile weight.
Or you want the weight of a pencil barrel profile but more rigid so you flute a HBAR.
Link Posted: 9/14/2004 9:52:31 PM EDT
^ Many excellent match guns have been built with fluted barrels. However we are usually talking about bolt guns with 24" or so barrels with a diameter of 1.25". This is a weighty piece of steel and often fluting is required to make weight restrictions. The unfluted barrel of the same diameter would be stiffer and likely more accurate. Improper, uneven fluting, or fluting a barrel with signifigant residual stress can be extremely detrimenatl to accuracy.

With that said your AR is pretty much limited to a 1'' pipe and not often do we go over 20". The barrels you are looking at are not Krieger, Shilen, or any other top line match barrel and I would be dubious as to how well the steel was stress relieved. I would go with an unfluted lightweight barrel or if your looking for accuracy a totally different barrel. I would also want to make sure my barrel was chrome lined if was being used for general purposes.
Link Posted: 9/14/2004 9:57:36 PM EDT
There is a reason why the newer 20" M-8 rifles are going to be coming with a fluted barrel.
Link Posted: 9/14/2004 10:23:41 PM EDT
I've got spiral flutes on my 20" and it cools much faster than any other barrel I've owned, of course I have never owned a non fluted equivalant so I guess that little tid-bit is irrelevant.
Link Posted: 9/15/2004 3:57:26 AM EDT
If you look on Bushmasters website there is a 1/2 lb weight difference between a 20" HBAR and a 20" Fluted barrel. Yes fluting will reduce the stiffness a little, probably a barely measurable amount. From talking to an Engineer here a work the barrel with the greatest cross sectional area will be stiffer. I say go for it the weight savings is worth it. Besides with barrels ordered from Bushmaster it is only a $50.00 option. As far a the heat thing goes I would say the greater surface area claim is true but it's effect might be hard to notice. Bottom line thin barrels heat up fast but also cool fast, thick barrel heat up slower but also take much longer to cool.
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