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Why is there an optimal barrel length?

11 Mar 2017
@ 11:08 am (GMT)

Benn Harvey-Walker

Hi All,

This is purely a technical question, possibly with no definitive answer, but I'm curious to know why various calibers are said to work best when fitted with a barrel of a particular length?

I know there are legal restrictions on minimum length and practical restrictions on maximum length, but why is it that, say, a 30-06 is said to work best with a 24-26" barrel?

I also understand that barrel length has an impact on achievable velocity, but velocity is not the sole determinant of accuracy. (I don't think I own a rifle that is most accurate at the maximum recommended load for projectile weight.)

Clearly there are many variables ... Barrel stiffness, heat management, recoil mitigation etc., but broadly, is a rifle going to be handicapped if the barrel length is outside the so-called 'optimum' and if so, why?




11 Mar 2017
@ 12:54 pm (GMT)

Lane Salvato

Re: Why is there an optimal barrel length?
A lot of it has to do with the original design or function of the caliber of which you speak. It's a good question.

I can't make a large, sweeping answer, but in the case of my favorite 25-06, a 24-inch barrel gives you a chance for developing full velocity and getting the most potential out of your rifle. It was the original barrel length for the original design. Same with the 270. It was originally designed with a 24-inch barrel.

Nathan talks about this in his books, and on some of his articles.
11 Mar 2017
@ 01:05 pm (GMT)

Nathan Foster

Re: Why is there an optimal barrel length?
Hi Benn, you have pretty much answered your own questions. All very true and based on good common sense.

Earlier in the week a reader sent me an email saying that folk put too much stock into optimum barrel lengths and that a short barrel can be highly effective. This is quite true.

He also included a test data sheet with several cartridges losing only 100fps when docked from 26 to 20 or 21". This was also valid- based on those test circumstances.

Here is some of my response, edited for this forum:

"The data you quoted is useful but there are a few traps with this. To begin with, the latest craze is docking short and suppressing. The velocities are OK and you do get a bit back with the suppressor- but the suppressors can only handle this for so long until they rupture which is very dangerous. The most recent presented to me was a 21" .300 Weatherby suppressed. I think both the smith and customer must have been equally foolish to not see what was going to happen to the muffler on that rig. It gets so bad that I keep wondering if these rifle stories are prank calls.

Suppressors and current trends aside, shortening up a magnum power rifle as per the data generally increases recoil via reduced weight and a rear heavy aspect. If the stock is of a Weatherby shape, the rifle will roll back and clock the shooter. If a brake is fitted to overcome these new problems, the noise is very close to the shooter.

The data also does not take into account specific loads where for example, you might use a very slow powder in the .300 to drive a 200-225gr at max possible speed. The quoted Whelen itself is a good example where folk may use H4895, Varget or 4350. You cannot predict just how each person will load their rifle. The slower the powder the worse the result in this less than efficient case to bore ratio. Even with H4895, there are some larger drops in velocity with the .35 bore if its docked too far. A short .35 is very useful but if you study my low velocity terminal performance, you will see that for best results you really are working in a window of say 2600fps to 1800fps. In this instance, 100fps can make a difference depending on how much reach is required.

What I am trying to say is that there are many factors to take into account. My main hack rifle is a short barreled .308 as it is so handy, effective and efficient. A short 7mm-08 can also be extremely effective.

On the other hand I like to keep my magnums long for various reasons. Having said this, I prefer a 26" .338 rather than a 32" monster than is impossible to carry any distance and poses a good deal of what you might call harmonic sag (length diminishing rigidity). At 26", 11lb and with a good stock design, I can shoot a 285gr pill at 2735fps accurately with or without a brake. By the same token, the .338's are not the fastest of cartridges. It is possible to dock too far and lose performance (mostly wind annoyances) at extended ranges with heavy pills.

As for the .35 bore, I am always weary of low impact velocity thresholds (see knowledge base - .35 Whelen).

Of course 100fps does not always make a great difference. Still, we each have various goals. I think the key is to be mindful of the wider range of consequences (especially if suppressing). A gunsmith might perhaps be best to keep on recommending traditional lengths as this does help minimize recoil and keeps the noise forwards which will ultimately mean that his customers shoot well which will reflect kindly on him. It will also allow the customers to trial a range of powders and bullet weights. Meeting the customers needs are much different to meeting his wants. Learning to differentiate between the two only comes with experience. Furthermore, you have to take each customer to the range to see this, not simply hand over finished rifles.

I am going to upload a video soon, designed to teach folk how to set up and shoot a Tikka magnum rifle. This is a rifle that many folk including "experts" simply can't shoot- they just don't have the technique for such a light rig anymore and so they never really see what it is actually capable of (making what people want- not what they need). If Tikka had opted for a medium weight 24" (WSM) or 26" (long magnum) tube on the non varmint magnums it would have been a lot easier on shooters."

End of edited email response.

If you ask a gun maker like Remington, you might generally find that they will try to match the barrel length to the type of powder that they think will most likely be used. These factors are balanced with portability. But not all gun makers think this way. Some may put portability ahead of burn rates. At other times, the gun maker (Remington included) may release a compact yet potent rifle just to see how it sells. But in most instances, sales of these are a bust and not simply due to velocity concerns.

As for your initial question regarding the .30-06, well, it all depends on how you want to use it. For a long while now we have been obtaining WSM performance from the 06 with a few tricks, aided by a long barrel, slow powder and heavy hard hitting pills. Not the nicest to shoot in a flyweight rig and certainly not optimal in a short barreled rig. Though all of this is subjective relative to one's goals and expectations. Much the same as how some think that even though the T3 Lite can easily be carried by a small child, it is still too heavy to be carried any distance by a fully grown man.

11 Mar 2017
@ 02:17 pm (GMT)

Benn Harvey-Walker

Re: Why is there an optimal barrel length?
Thanks Nathan & Lane. I appreciate the responses. Both help with my understanding.

The question arose because the Ruger 30-06 I'm about to take possession of is stock with a 22" barrel ... Presumably sold this way for ease of handling and to keep weight down on a hunting rig?

My initial thought was to maybe re-barrel it to get into the "optimal" zone, but then I couldn't think of a compelling reason to do so if the rifle is accurate (for me, sub-0.5MOA).

So, in my case, it's not about going as short as possible. Just wanting to understand possible limitations of the rifle in question.

Thanks again!

12 Mar 2017
@ 09:47 am (GMT)

Nathan Foster

Re: Why is there an optimal barrel length?
This is an entirely different situation. You bought the rifle based on a good deal and possible accuracy potential. Ruger selected a portable barrel length as you suggest. It is what it is.

You have a good mag length which is a great plus. You may lose up to 70fps with the heavy pills and slow powder due to barrel length but this should be neither her nor there. The weight of the Rugers is neither light nor heavy. These rifles can be highly rewarding to work with, starting out as very poor shooters, but can shoot very well depending on how much work you are willing to do. Thats where the rewards are. Putting a new barrel on this rifle might be nice but kind of goes against the originally good deal. So I think we need to put barrel length aside in this instance.

A customer once sent me his dearly loved yet beat up old Parker Hale (K98). He did not want a new barrel, he just wanted to see what the existing rifle was capable of. I bedded it, worked the trigger, gave the bore a severe birthday, fitted optics, loaded it with 2208 / varget and 150 and 165gr pills (these days I would use 168gr as a light weight load). I worked up basic loads, then took it straight out to 600-700 yards. I took a short video of sooting the rifle at these ranges with my clunky mini DVD camera, then sent the rifle and cut DVD back to the customer. Not exactly a custom rifle but it still felt good to take this rifle, the sort that generally gets sighted in over the bonnet of a truck (minute of A4) and tune it up. This is however risky as generally speaking, the bores cannot be revived at this age. So in this case, a mix of good luck and management. It could really have gone either way.

We have tricked up a number of Ruger rifles over the years. All highly rewarding experiences, even if frustrating at times.

The .30-06 has a long effective range regardless of whether it is slightly handicapped by barrel length or a short throat or even a short magazine. It is simply a wonderful cartridge to work with. A medium weight 24" barrel can help with recoil management while squeezing a few more FPS out of the rig but your rifle should be just fine.
13 Mar 2017
@ 06:10 pm (GMT)

Warwick Marflitt

Re: Why is there an optimal barrel length?
I thought it was to do with the "Bore expansion ratio" Diameter of the bore x length of barrel
just as the cylinder in a car engine has a "Swept volume ratio " diameter of the cylinder x the distance the piston travels.

Plus the volume of the combustion chamber. Something seams familiar here
Combustion chamber volume = Case volume
Swept volume ratio = Bore expansion ratio

A car engine is a bit like a gun/rifle.
the piston = the bullet
the cylinder = the barrel
the combustion chamber = the cartridge Case
the fuel and air mixture = the powder charge
In an engine the amount of fuel in the combustion chamber is relative to the swept volume . We only need the fuel burning and combustion forcing the piston on the down stroke. Having powder burning after the bullet has left the barrel is just making noise and wasting powder. So the amount of powder/Fuel burning speed will have to be shorter/Quicker in a short barrel to build pressure before the bullet (cork pop's) out the end of the barrel or piston arrives at the bottom of the power stroke.

Hopefully you'll all be totally confused and Nathan will be laughing and shaking his head.


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