Is There An Optimum Barrel Length for a Caliber?

Most discussions that open up with the question "what barrel length should I get" yield a variety of answers. Most are "depends on what you want to do with it" or "I have a barrel that's xx" long and it does all I want it to".
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This morning I got to thinking that there has to be some data out there that supports an "Optimum Barrel Length" theory. I've seen references to X number of "calibers" in barrel length where the diameter of the bullet is divided into the barrel length, or bullet is multiplied by a proven factor to determine barrel length.

Is there such a magic number? If so, what is it for the common calibers used today in LR and/or Precision shooting (.223, .243, 6.5 Creed/.260/6.5X47L, .308, etc).

I know that everyone who's built a rifle has relied on recommendations of friends, forum members, gunsmiths, articles, etc to select their barrel length but I was just wondering if their was a scientific formula?


What say you all?

Deadshot,

That's a complicated question. I'm not expert by any means, but I will give you my 2 cents on the topic. There certainly is an optimum length, but I guess you have to ask, "What are you trying to optimize?" If you are talking about reaching the highest velocity, a barrel will often tend to be longer than most prefer. I hate to repeat what you said, but it really does boil down to what you want to do with it. If you want to squeeze every bit of velocity out of a cartridge, for something like long range shooting, the barrel will be long.........at least with the cartridges I have used. For a hunting rifle, which may be carried, a shorter, lighter profile is more practical, and the rifle will be easier to hold shooting offhand and better balanced. The bullet may be slower from a shorter barrel, but not by enough to adequately kill the critter the hunter is shooting, at the range he intends to shoot. Also, you reach a point that there is very little to be gain by going longer. The bullet may still be increasing in speed, but not enough to justify another inch of barrel, as other factors like harmonics come into play. Barrel thickness is another matter to consider. All other things equal, a bull barrel will shoot better overall than a skinny barrel of the same length. A shorter barrel of the same profile should shoot better than a longer one, although velocity is sacrificed. Heavy weight barrels resist accidental movement by the shooter when shooting on a bench. Thicker barrels take longer to heat up, which may allow for better groups longer. But, I have gotten off on a tangent with the weight thing, as I think you are concerned about velocity.

I guess it all depends on how much velocity you are trying to achieve for what you are doing. I really believe for a 5.56mm/.223 rifle, the optimum length is 20", especially with an AR-15 type rifle. (However, you can achieve higher velocities with a barrel longer than 20".) I recently decided to build a 5.56mm 16" AR-15 lightweight upper for a vehicle/carry rifle. The reason being that it will be more handy in that situation than a rifle with a 20" barrel. Yes, I will be sacrificing ballistics, but for a short range SHTF rifle, it really doesn't matter to me.

Anyway, there is data out there, especially for velocity per inch of barrel..if that is what you want.

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Originally Posted by NicfromAlabama

Deadshot,

That's a complicated question. I'm not expert by any means, but I will give you my 2 cents on the topic. There certainly is an optimum length, but I guess you have to ask, "What are you trying to optimize?" If you are talking about reaching the highest velocity, a barrel will often tend to be longer than most prefer. I hate to repeat what you said, but it really does boil down to what you want to do with it. If you want to squeeze every bit of velocity out of a cartridge, for something like long range shooting, the barrel will be long.........at least with the cartridges I have used. For a hunting rifle, which may be carried, a shorter, lighter profile is more practical, and the rifle will be easier to hold shooting offhand and better balanced. The bullet may be slower from a shorter barrel, but not by enough to adequately kill the critter the hunter is shooting, at the range he intends to shoot. Also, you reach a point that there is very little to be gain by going longer. The bullet may still be increasing in speed, but not enough to justify another inch of barrel, as other factors like harmonics come into play. Barrel thickness is another matter to consider. All other things equal, a bull barrel will shoot better overall than a skinny barrel of the same length. A shorter barrel of the same profile should shoot better than a longer one, although velocity is sacrificed. Heavy weight barrels resist accidental movement by the shooter when shooting on a bench. Thicker barrels take longer to heat up, which may allow for better groups longer. But, I have gotten off on a tangent with the weight thing, as I think you are concerned about velocity.

I guess it all depends on how much velocity you are trying to achieve for what you are doing. I really believe for a 5.56mm/.223 rifle, the optimum length is 20", especially with an AR-15 type rifle. (However, you can achieve higher velocities with a barrel longer than 20".) I recently decided to build a 5.56mm 16" AR-15 lightweight upper for a vehicle/carry rifle. The reason being that it will be more handy in that situation than a rifle with a 20" barrel. Yes, I will be sacrificing ballistics, but for a short range SHTF rifle, it really doesn't matter to me.

Anyway, there is data out there, especially for velocity per inch of barrel..if that is what you want.

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You are in the neighborhood of what I was asking. The key statement in your answer was "you reach a point that there is very little to be gain by going longer." That's exactly what my question was centered on. I realize that harmonics, etc, play a role but those can be changed with barrel contour and other time honored methods.

As for data on velocity per inch of barrel, I've read a lot on various tests where barrels were hacked off an inch at a time and velocities measured. Most of them just used a chronograph to measure speed from a single barrel. Because speed is based on pressure behind the bullet, and pressure is a function of heat, volume, etc, certainly somewhere someone has come up with a formula using all the various known parameters to calculate that optimum length.

I have read some information regarding large bore rifles like artillery and naval rifles. For them, optimum length has long been listed in "calibers" but the only place I've seen that term used for small arms is with Lapua TRxx rifles.

There is no universal answer to your question, unfortunately. Nic did a nice job, but to your second comment and why there is no single answer:

"Because speed is based on pressure behind the bullet, and pressure is a function of heat, volume, etc, certainly somewhere someone has come up with a formula using all the various known parameters to calculate that optimum length". There are always variables which make that impossible, without an acceptable tolerance. All powders should be expected to vary by 10% between lots. I've pressure tested less, but pressure tested much larger swings between lots. Likewise case capacity, and primer output will have variances. Increased fouling and barrel dimensions are all variables. Likewise every powder will have it's happy place in terms of operating pressures in any given cartridge. But the newer progressive powders actually change their burning curve, as pressures change. So the amount of effective push being accomplished by the duration of the curve changes as well.

So as Nic said, what is it that your are after? Most Velocity at any cost? Highest velocity per dollar spent?

For the 6mm ppc it is 21 3/4"

An interesting article on precision shooting is Secrets of the Houston Warehouse. Anyone who is interested in precision shooting should read it.

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Originally Posted by m12lrs

For the 6mm ppc it is 21 3/4"

An interesting article on precision shooting is Secrets of the Houston Warehouse. Anyone who is interested in precision shooting should read it.

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I have reat that article, from beginning to end. That was one of the experiments I was referring to when I mentioned just cutting off barrels. Those who conducted this experiment found that the best length for that barrel contour was 21.75" but they focused on accuracy rather than speed.

Perhaps I'm traveling down a rabbit hole that has no answer. I think Darkker hit it when he stated "max speed at any cost". It takes speed to get to long distances. Speed flattens trajectories. Speed exposes a bullet to the effects of wind for a far shorter time.

I guess this comes from a conversation with a Palma Shooter and team coach where he told me that their team settled on a single barrel length and speed so their spotters could communicate corrections uniformly. This quest for an optimum barrel length is just a point on my 'thinking path".

Speed is far from everything. If so then why was the change from light, fast bullets to heavy, slower, high BC bullets when the long range craze hit?

Accuracy is more important than ever in long range shooting.

Like everything else in accurate shooting consistency is the key. Just like the palma crew using the same load. Consistency!

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Originally Posted by m12lrs

Speed is far from everything. If so then why was the change from light, fast bullets to heavy, slower, high BC bullets when the long range craze hit?

Accuracy is more important than ever in long range shooting.

Like everything else in accurate shooting consistency is the key. Just like the palma crew using the same load. Consistency!

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I understand this. However if someone could figure out how to make a long, heavy, bullet, go faster, I'm sure they would.

I'm trying to just focus on whether or not their is a magic relationship between "caliber" and "barrel length".

I do know that there are ways to gain "consistency" with speed. Barrels have various harmonic nodes and most shooters can't get enough speed out of a given caliber reach the higher nodes, those above the "third harmonic".


Is there a "formula" that says a 20" barrel is "optimal" for a .223 and says that 45" is "optimal" for an M-2, 50 cal Machine gun (along with all the in-between calibers).


I'm really not stubborn, just focused on one item;)

Here's a statement from a Wikipedia article on Caliber.

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The length of artillery barrels has often been described in terms of multiples of the bore diameter e.g. a 4-inch gun of 50 calibers would have a barrel 4 in x 50 = 200 in long.

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I realize they are talking about some huge "Rifles" compared to what we all own and shoot but is the principle somewhat similar?"

A .50 Cal M-2 with 45 " barrel has a bore/caliber ratio of 22.5. Most other calibers share a "ratio" of 4-6 or thereabout.

Long heavy bullet going faster

Well they have. All kinds of wildcat magnums out there

If you do a little research you will find short range benchrest they use short barrels

But the real 1000 yd shooters use 30" and longer straight taper barrels unless they are limited by a weight class.

All depends on what you want to accomplish

If you are trying to justify a pre-conceived opinion i am sure the information is out there somewhere to back it up

All kinds of things work if it is done right.

Among countless other bits of information reputed to be facts online, and ever since I read everywhere I looked on the internet that a 25-06 needed a 26" barrel, and then bought a 22" sporter that shoots bug holes and has been piling up up game for decades. I learned that reading the info others have provided, and then throwing the book out the window was the best approach for me.

I'll answer your question with a question...

Is the optimal barrel length for a 22LR and a .22-250 Rem the same? How about the optimal barrel length for a .300 Savage and a .300 RUM? I think you'll find the answer is a resounding no. Much more to optimal barrel length than just bore diameter - one has to factor in case capacity, case efficiency and the pressure curve - just to name a few of the many variables.

Then of course you have to ask yourself, "Optimal for what?" Optimal for velocity? Optimal for accuracy? Optimal for fast handling? Optimal for all of the above?

How about barrel length and.powder choices?

You guys are looking for something that simply doesn't and can't exist. There are just way to many variables involved. And again, the definition of "optimal" is going to change depending on what you're after (accuracy, velocity, handling, sight radius, etc).


Why do you think that when these writers do their tests to determine the "optimal barrel length" for a given cartridge they stick with the same load? It's simple, doing so greatly reduces the number of variables in the question. However, in doing so they also make the results all but pointless because all they have done is determined the optimal barrel length for that specific load in that specific barrel. Change the barrel, powder type or charge weight, or the bullet brand/type/weight and that "optimal length barrel" is no longer optimal for the new combination.

One could literally spend a lifetime and several fortunes on barrels, ammo and reloading components trying to find the definitive optimal barrel length for a given cartridge and never come to a definite conclusion. For starters they would die before they had enough time to fully test all the possibilities. Secondly, each and every barrel is a unique animal unto itself so just when you think you're getting somewhere you shoot out a barrel and have to replace it and start over from scratch. Hamster meets wheel.

I believe you need barrel length in a mag to burn all the powder, I do believe that !!!

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Originally Posted by deerhunter99

I believe you need barrel length in a mag to burn all the powder, I do believe that !!!

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You would be surprised if you had Quickload...

According to QL, Running numbers for a 300winmag shooting 208gr bullet, 3.500" coal,... some powders like Retumbo and RL-25 don't need more than 20" of barrel to fully burn if detonated at peak pressures (60 to 63,000 psi chamber pressure). And H-4831 powder needs 34" of barrel to fully burn in the same rifles with the same bullets and chamber pressure.

This would explain why at dusk I have never witnessed muzzle flash with Retumbo or RL-25, and seen flames with H-4831.

I believe there is an answer to your question. Unfortunately the answer is based on statistics, and honestly I hate statistics. My thought process; every gun has the same purpose, that is to hit a target at a specified range. If you know your worst case target and your worst cast range, smallest and furthest, then you will know the group size that is an acceptable max.
Now look at your specific cartridge, specifically the consistency of the powder that you will or may use. That powder may only have a max SD of 10 fps which would equate to a specific vertical dispersion of impacts down range.
Now you have a design criteria and one input variable identified. Last is the barrel. Some of your allowable group size has been consumed by your powder SD, the rest can consumed by your barrel/action. Looking at the amplitude of barrel deflection you can determine the ratio of barrel diameter vs length that maintains your target size at a given range.
At this point barrel length is just a result.
After that, I would rather just keep cutting an inch at a time off my barrel until I'm happy. I hope you find this response amusing as I intend for it to poke fun at the complexity of such a question.

Sent from my Pixel using Tapatalk

Here is a tool that will help calculate the relationship between barrel length and velocity: http://shootersnotes.com/calculator/velocity-estimator/

You can generate data to draw nice curves showing velocity as a function for different calibers and bullets.

The laws of physics, however, tell us that the rate of velocity gain diminishes as the barrel length is increased.

BOTTOM LINE:

1. Decide how you plan to use the rifle --
   
   1. busting through brush or clambering up and down steep canyons suggests a short, light barrel -- velocity is a secondary consideration
   2. long range varmint shooting on windy plains suggests a longer barrel with perhaps a heavier contour
   3. long range competition from static positions suggests the longest barrel practicable to gain velocity for wind bucking. (High BC trumps velocity.)
   
   
2. You will need to balance your strength and stamina against that extra 20 - 30 fps per inch of barrel. Remember, even the strongest folks find that fatigue from toting heavy objects disrupts fine motor skills.

I think I'm with you OP - it'd be nice to know at what barrel length the velocity increase really tapers off. From my research, it isn't a huge taper with most rounds. It's not like it hits max velocity then stays the same and then that's the ultimate barrel length... it slowly tapers off. It seems like most calibers expect 25FPS difference per inch until you hit 24", and then that number of FPS slowly decreases per inch. Because of this, I think for a bolt action in a standard caliber 22-24" is about right. 20" is okay if you're trying to keep it short/maneuverable. I chopped my .308 down to 18" and still ring steel at 1000. Just like all the other responses... it depends. Cartridge, bullet, powder, twist rate, etc etc etc. I would like a 6.5 creed and was leaning toward 24" Sendero profile. If I went with a thicker profile I'd probably go 22 to save weight.

I went with a 25" barrel on my 260 rem as I read somewhere that when using H4350... The powder is still burning when the bullet leaves a 24" but powder is finished burning in a 26"... So I split the difference, trying to remove any "bullet drag" but still get max velocity.