been reading about the 6.5 grendel and have been very impressed. If barrel length could be increased it may compete with the 6.5x284!? I have never seen an AR with a barrel longer than 24 inches. Is there a max. length and why?

no semi auto guys out there? should i try somewhere else?

I have a Colt 6724 (Colt Acurized Rifle) with the 24" BBL. I have no experience with anything longer. Try http://www.AR15.com good guy's there..

Barrel length, to a large degree, is a product of chamber pressure and burning rate of the powder being used.

As the bullet moves down the barrel the area in the barrel behind the bullet...that the expanding gas is trying to fill...gets larger (longer.) In an overly long barrel that area may become so large before the bullet leaves the barrel that the expanding gas starts losing its acceleration effect on the bullet because it is doing nothing but trying to fill up the area behind it that is offering no resistance.

good point collins ill do that. rick has a good point also. that is a small powder charge. hmmm.

An easier answer is that the length of a barrel should be just long enough to allow the most efficient use of the expanding gas to accelerate the bullet to its maximum velocity.

In modern centerfire rifles that length is usually somewhere between 22 and 26 inches depending on the cartridge.

standard length of F class 6.5x284 rifles is 28-29 inches. thats why i wondered why not in an AR-15. I need to break down and get a 6.5x284. then i can stop trying to find something to get around it.

A very common misconception is that a longer barrel allows better burning of the powder. Not true.

With a properly designed load there will be nothing in the barrel when it is fired except the bullet and the expanding gas. Combustion of the powder is supposed to happen in the case...not in the barrel.

Rick,
normally we see perfectly eye to eye, on this one we don't.



barrel length is a function of expansion ratio and "over/under bore"

an overbore round has an incomplete burn, pretty much no patter what the length.

Powder burns ALL the way down the rifle.. powder fast enough to burn in the case is like blue dot and I don't even think that would work in most cases... that's the advantage of "slow" powders.. they litterally leave the case and burn all of the way down the barrel. This is why you can't just cram a 308 full of h1000 and expect it to go faster

a well balanced round will continue to gain velocity generally to 32 inches.. but the amount of gain (fps) per inch has a sweet spot from 18 to 28 inches... Oh, the muzzle flash is much more at 18" (burning powder).. for example.. a 30/30 can get along just great at 20"... but a 30/378 just goes faster at 28" .. and either would go "faster" at 32"... but the fps per extra inch falls off (expansion ratio and "bore")

Barrel length is always a compromise.. weight, handling, "looks".. I don't think i've seen a long barreled ar either..

jeffe

jeffe,

Hey, if everyone always agreed with me I would start worrying about myself...and them!

You are right though, on this we do disagree.

Expansion ratio and barrel length are interdependent...and overbore is over bore no matter what those other two may be. Gas expanding and escaping ahead of the bullet (which always happens to some degree upon ingnition) momentarily increases the volume the expanding gas is trying to fill, which lowers the pressure pushing on the bullet. If the bullet is the proper size it re-seals the bore as it enters the rifling and the pressure spikes back up because once again the bullet is the path of least resistance for the expanding gas.

Expansion ratio is obtained by dividing the volume of the space in the case (gee that rhymes !) with the bullet seated in the case, into the the the combined volume of that space (without the bullet seated) and the bore. That gives you the area (volume) that the expanding gas has to expand into prior to the bullet leaving the bore. This is the determining factor for muzzle velocity, not for barrel length.The lower the ratio the higher the velocity because the expanding gas will have a smaller volume to fill which will increase the pressure pushing the bullet down the barrel.

Within the reasonable lengths for a shoulder fired weapon the expanding gas will continue to function and increase the velocity as longs as the bullet is in the barrel. The “ideal†situation is when the only thing in the barrel after ignition is expanding gas (not burning powder) and the bullet. See Robert Rinker’s book Understanding Firearms Ballistics.

A certain amount of expanding gas will always blow by the bullet on ignition/combustion and I have seen this in those high speed camera tests I spoke about on here a few months ago. It was amazing to see how the gas that beats the bullet out changes from shot to shot and from weapon to weapon. Watching that made me wonder how in the hell anyone could ever design a muzzle break that allows any degree of repeatable accuracy.

Anyway...all this “heady†stuff is fun to jaw about, but in the real world little of it has much practical value to the average shooter. “I pull trigger...gun go boom...I am happy man!â€

interesting debate.....please allow my .02 here.....

powder burns at it's prescribed rate and under the pressures generated....a very complex formula I believe. The powder can still be burning when the bullet exit's the barrel.....I believe this is true with the large handgun rounds. When these same rounds are used in longer barreled rifles they generate a lot more velocity with the same charge.....and this is because they are still adding (fuel) pressure to the bullet as it's running down the barrel.

There comes a point however when the fuel is burned and the pressure drops and the velocity gains are quickly diminished because the pressure drops dramatically.

I believe the powder is burned in the case and the gasses push the bullet with the F=MA equation in such a manner that the extreme max pressure does not exceed a certain level and as much pressure is kept on the bullet all the way down the barrel.

Small and fast burning charges have no advantage in long barrels.....anyone want to guess the gains in the .22 LR past 20" of barrel?

I believe most shotshells gain no velocity past 21" of barrel...(this is from tests at Remington)...they have small and fast charges.

We also quickly reach diminishing returns when the bullet is extremely fast as there is so little time for the expanding powders to further act on the bullet. A 7MM STW bullet spends a very small amount of time in that last two inches of steel. The resulting gains are minimal IMO.

There's a reason why 16" naval guns have very long barrels......in fact they really don't shoot very fast... They burn extremely slow powders and in huge quantities.

IMO the small and fast powder charges have no value in long barrels unless someone has a accuracy edge in mind....and that's questionable too.

The only cartridges that can really benefit from long barrels is the slower (and very large) magnums.....such things as the .505 Gibbs and cartridges like the 416 Rigby where there's a lot of room for slower powders to keep the pressure on the bullet longer and the velocity is still slow enough to allow time for the pressure to act.

In short.....long barrels for sporting rifles are for balance and looks more so than performance.

Some folks like them as they "believe" there's something extra in them.....and in fact there is....but very little.

vapo,

From my conversations with him, I believe that Tom Burgess will soon be posting info related to this topic. It’s fascinating stuff...and I have to admit that much of it goes right over my head.

I still don’t subscribe to the “theory†( which is all any of this really is)that powder is supposed to be burning all the way down the barrel. This may happen to some degree, but it is not the ideal, nor the most efficient way to design a cartridge. Combustion of the powder is most efficient when it takes place inside the cartridge case...or in the case of Naval guns, inside the chamber, and the only thing traveling down the barrel is the bullet and the expanding gas pushing it. Just think of the never ending, and unpredictable variables that would occur if the powder was blown down the barrel upon iginition and was free to ignite whenever and wherever it wanted to behind the bullet? Sort of like the guy that named his dog “STAY†and wondered why the dog always stutter stepped when he called him...â€Come...Stay..Come Stay!â€

The reason 16 inch Naval guns use “slow†burning powder is to keep the pressure from blowing the damned things apart. 16 inch guns have to send a 2,700 pound projectile down a 60 foot barrel at 2,800 fps. It takes a powder charge of 576 pounds (about 4 million grains!) to do that.

I’ve been close (about 500 yards) to the receiving end of Naval gun fire and I will tell you that it is the most frightening and awe inspiring experience you can imagine. I can’t even begin to imagine what it is like to be near one of them when they fire. On the other end of the game you would never know what happened!

quote:

From my conversations with him, I believe that Tom Burgess will soon be posting info related to this topic. It’s fascinating stuff...and I have to admit that much of it goes right over my head.

Always a pleasure to read opinions of Mr Burgess.....or anyone else that can muster logic and learning.