Optimal barrel length for 5.56 mm

[DMK]

Fluting a barrel must have some benefit or everyone of us (including me) who's paid that extra $50 bucks to get it is a flaming maroon suckered in by shyster gun company marketing weasels.

It does have a benefit. It makes the barrel lighter without giving up as much stiffness. In the right context, it is not snake oil. But it does not defy the laws of physics either.

A fluted barrel is not stiffer than an HBAR of the same diameter and length. It is stiffer than a thin barrel of the same weight and length.

Again, everything is a compromise. Know what you are giving up to get what you want.

 

[RecoilRob]

I have a 16" Colt w/pencil barrel and a Bushmaster Varminter w/fluted 24" heavy contour. Chose the fluted version because the un-fluted was too heavy for me. Fluting allowed me to have the extra barrel length without being so front heavy that the rifle was uncomfortable to shoot offhand.

Oh, Werewolf....you are misreading the article referenced previously. Look at the graphs for .8 dia barrels, both fluted and non-fluted. The fluted one is MUCH less stiff.

The computer model only projected in the vertical plane...not very realistic. And, the full barrel diameter was left in place by placing the flutes to the sides....again to retain maximum rigidity in the vertical plane. If fluting made the barrel 'more stiff', wouldn't it seem reasonable that the flutes could be placed anywhere to achieve this benefit?

The conclusions drawn by the author are also a bit mis-stated. His contention that the fluted barrel is more accurate is based on the assumption that they are the SAME WEIGHT. This means, of course, that the fluted version must be greater in diameter....hence the greater rigidity.

 

[jmr40]

Yes, fluting a barrel will make it more rigid. A 1/2" pipe is much harder to bend than a 1/2" solid bar of steel because of the greater surface area from the inside of the pipe. This can easily be proven. Steel I-beams are used in construction because the greater surface area makes them much stronger than comparable sized solid beams.

 

[RecoilRob]

Oh boy, please don't take this the wrong way, but I hope you aren't designing any bridges or the like anytime soon.

If a steel pipe gets its' strength from the 'greater surface area', would a 3" pipe with a .005 wall thickness be stronger than a solid steel 3" rod? The hollow pipe has MUCH greater 'surface area' but, I think you would agree that the thin steel would buckle pretty easily.

 

[SaMx]

jmr40, I'm telling you from the point of view of an engineering student, that you are wrong. Adding surface area does not make a rod stiffer, and a pipe is not stiffer than a solid rod of the same diameter.

I beams are used because they are almost as stiff as a solid rectangular block of steel, but they are much lighter.

 

[SaMx]

EDIT: weird double post.

 

[SnakeEater]

I'm hearing the 75gr or 77gr loads may actually fragment out to 250 yards if you get the velocity right.

 

[SaMx]

http://www.fulton-armory.com/fluting.htm

Conclusion: the fluted barrel is much lighter, much less rigid, and has much more surface area than a solid barrel of the same diameter.

Conclusion: the fluted barrel is significantly more rigid, and has much more surface area than a solid barrel of the same weight.

fluting will reduce the weight and rigidity of a bull barrel, but are still more rigid than a thin lightweight barre.

 

[no_problem]

14.5" the optimal length?

Of all the barrel lengths in the world, why did the Army's M4's choose to use 14.5" barrels?

 

[jmr40]

SaMx, the link you provided proves that I am wrong. Thanks.

 

[Gtscotty]

*Whips out book* Well actually....

Interesting, most of this discussion on fluting and stiffness transpired while I was at the library studying for my Deformable bodies final . I beams and wide flange beams are not used because their added surface area contributes to stiffness. The youngs modulous (stiffness) multiplied by the moment of inertia = a term called the flexural rigidity, aka the resistance of a beam to bending (or barrel whipping). A bull barrel will be stiffer than a fluted barrel with the same nominal diameter, but the fluting allows you to get an increased moment of inertia for a given weight. This in turn contributes to a higher rigidity. I know people have already effectively said this, but after hours upon hours of studying this crap, I could not pass up a chance to regurgitate it.... it makes me feel better about the time spent . (Btw, I beams are used because for a load coming from a singe direction, they too effectively maximize the rigidity for a given weight.)

 

[DMK]

I'm hearing the 75gr or 77gr loads may actually fragment out to 250 yards if you get the velocity right.

75gr or 77gr loads will generally fragment reliably out to about 200 yards with a 20" barrel and 160-175 yards with a 16" barrel. I believe they need to be in the 2300-2400fps area to reliably fragment. They may fragment farther out, ...or maybe not. So with a 24" barrel, you might could push that out to 250ish yards.

 

[Bartholomew Roberts]

Of all the barrel lengths in the world, why did the Army's M4's choose to use 14.5" barrels?

The original Colt Commando started with a 10.5" barrel and a moderator that reduced the sound to normal 20" rifle levels as well as creating additional backpressure/dwell time that helped the rifle function. The 10.5" didn't function as reliably as desired (short dwell time, big gas port) so they lengthed the barrel to 11.5" to get better reliability and left the moderator in place. The moderator was considered an NFA item and was a pain from a record keeping perspective; but if you removed it you lost some of the additional backpressure that helped the weapon function. One way to get around that is to lengthen the barrel past the gas port so that you have roughly the same dwell time as the rifle (about 4" of barrel). This gives you a 14.5" barrel.

 

[MechAg94]

rods vs tubes
In bending a rod or tube, the materal on the outside diameter sees most of the stress. The material in the middle is doing very little and might actually add to failure by cracking or deforming. So, a thick tube will retain most of the strength of a rod (since the outside diameter does most of the work anyway )with a lot less weight. How thick the tube is depends on your application.

Werewolf is mostly right but leaving out some details that confuse the issue. A fluted barrel is not stronger or stiffer than a non-fluted barrel of the same diameter. It may be stiffer than a smaller diameter barrel of the same weight. In other words, fluting lightens a barrel while maintaining much of the strength and stiffness of the heavy barrel.

 

[jkingrph]

Steel I-beams are used in construction because the greater surface area makes them much stronger than comparable sized solid beams.

I disagree, I beams are used because the shape gives them probably more structural strength than a comparable sized solid beam. Consider the weight of a solid beam, in a span situation it would have more tendency to bend( sag) of it's own immense weight than an I beam of the same "size", if by size you mean cross section demension( L x H) . You also must consider web thickness, beams can be made with the same Lxh but with different web thickness for different strengths. Almost any I beam of a comparable weight for the same length will be much stronger than a solid beam.

 

[SaMx]

yes, that's true, but the lower sag comes from lower weight. I'm not talking about an I beam of the same weight as a solid beam, I'm talking about an I beam of the same dimensions, as if the sides of the beam were filled in.

 

[MechAg94]

That is because the flats on the top and bottom are what hold the load. The fact that they are separated by the web gives them better load bearing ability without the high weight. I don't think the web itself sees a lot of stress.

It has been a long time since looking at I-beam stress. Not something I have seen since college.