Finding out the perfect barrel length for a cartridge..

[brentn]

Just curious, but with modern smokeless its a known fact that gases are expanding even when the projectile leaves the barrel, constantly accelerating the bullet until it leaves the muzzle.
Lets take .223 for example, is it possible to determin what length of barrel it would take before the gases are no longer expanding, accelerating the bullet? At that length, would that be the 'perfect' barrel length for that cartridge?

thx

 

[Sniper4Life]

Good question... i dont know about the .223 but a gunsmith which I respect greatly told me that the ideal barrel length for the .308 was 24 or 26in.

 

[eliphalet]

Different powders would make for different results.

 

[Harley Quinn]

It is not so much finding the correct barrel length, but dialing in a load to fit the length of the barrel. As has been mentioned by "eliphalet".

It is pretty much a given that many of the "Mil spec" was dialed into the barrel length during the course of developement.

Similar to other projects by the leading gunmakers.

So what are you using it for and why?

Regards

 

[rcmodel]

Not exactly what you are asking, but interesting reading non-the-less.

http://www.accuratereloading.com/223sb.html

http://www.loadammo.com/Topics/October05.htm

At what point there was zero further velocity gain is unknown to me, but it would be more barrel length then you would be willing to pack around I betcha.


rcmodel

 

[The Annoyed Man]

My 700 VSF in .308 came from the factory with a 26" barrel. Now, it may not be the best barrel that money can buy (although it not bad), it seems silly to me to contemplate the idea that Remington would have knowingly fitted one of their models with a barrel that will actually slow the bullet down before it exits the muzzle. Therefore, it is pretty certain that a 26" barrel is not too long for the .308 round in factory loads. In fact, I'll bet that 26" isn't even at the velocity limit. I'll bet you could add at least another 2"-4" before adversely affecting the velocity, but Remington sticks with a 26" barrel as a compromise because the rifle will become less handy with a longer barrel.

That's just my guess.

 

[DnPRK]

Palma rifles have 30" barrels to get the most of 155 gr Sierra Matchkings

 

[Float Pilot]

Different powders, different rifling twist and different bullet weights / shapes all come into play. As does free boring and chamber dimentions.
So does your brass selection and the depth of your bullet seating. The loads I list below worked because the old virgin WW brass held a little more powder than the other brass I tried.

For example, back in the early 70s, shorter heavier barrels were a rage for accuracy. So when I rebarrel a M-98 (1935 era DWM) I used a Douglas #5 taper (at least what it was back then) with deep 1 in 10 rifling. The barrel was (is) 20 inches long.
I reamed the chamber slowly until I had a very tight match grade fit. The caliber was 7 x 57mm Mauser.
My desired bullet weight was 150 to 160 grains.

I still have that rifle.

Here are some chronograph results. (I use two chronographs mounted in-line for double readings)

These were experimental loads in my rifle loaded to certain OAL . Do not use my loads in your rifle!!

20 inch Douglas barrel 7 x 57mm, DWM Mauser M-98 action
Brass WW virgin
Speer 250 primers.
Powder IMR 4350
Bullet 150 grain Nosler Ballistic Tip 7mm (284 caliber)

49.0 grains = 2730fps
49.5 grains = 2837fps (super accurate load 10 shots into one 5/8 inch hole)
50.0 grains = 2821fps (YES THE AVERAGE WENT DOWN) 1 inch group
50.5 grains = 2841fps 3/4 inch group.
50.7 grains = 2880fps 1.5 inch group hot load, dirty case, cratered primers

Now the old Hronady book shows about the same velocity using a 24 inch barreled M-70 and IMR 4350. But the Barnes book does not go that high with 4350 and uses RL-19 and a 24 inch barrel to barely get 2800 fps.
The od Sierra book used a 29 inch barrel for their loads. BUT THEY USED A WEAKER 1895 Mauser. So their loads are pretty light.
The Nosler book uses a 24 inch barrel, but maxs out at 2726 FPS using only 48.0 grains of IMR 4350.
Now the newer Speer book list my 49.0 grain load from a 22 inch barrel and they got 2727fps with a 145 grain slug instead of a 150. So a lighter bullet and 2 inches more barrel did nothing for the load.

So anyway, there are all sorts of issues that make a difference in your actual velocity and accuracy.
According to several supposed experts, I should not be obtaining the velocities that I obtain from my short barrel Mauser. But I do.

On the other hand ,I had a 30-06 M-70 Stainless Classic (24 inch barrel) that had terrible velocities and could not shoot a group under 3 inches no matter what I did with the BOSS system stuck on the end of the darn thing. That thing was sold as fast as possible.

 

[JohnBT]

"its a known fact that gases are expanding even when the projectile leaves the barrel, constantly accelerating the bullet until it leaves the muzzle. "

I'm not certain that's true for a long barrel. The gases are expanding, yes, but likely not enough to provide constant (or increasing) acceleration. With the longer barrel they could be expanding at a much lower rate than just after ignition and the bullet could be slowing somewhat while passing through the last couple of inches of the barrel.

This does in fact happen in standard velocity .22 rimfires with 27" or 28" barrels. By keeping the bullet subsonic the accuracy is improved. Come to think of it, it's been demonstrated by cutting a barrel down an inch at a time that a .22 LR achieves maximum velocity out of a 16 inch barrel. Not good for accuracy necessarily, but good for velocity.

John

 

[Father Knows Best]

it's been demonstrated by cutting a barrel down an inch at a time that a .22 LR achieves maximum velocity out of a 16 inch barrel. Not good for accuracy necessarily, but good for velocity.

Not sure I agree with that. Rimfire target matches are typically won by rifles with short barrels, because they are more accurate than longer barrels. Check out a typical rimfire silhouette competition rifle, for example. It will have a 16" barrel, but with a long barrel extension that provides for a much longer sight radius. The reasons are: (1) a shorter barrel is stiffer than a longer barrel of the same weight, which improves accuracy; and (2) greater accuracy results from decreasing the time between tripping the sear and the bullet leaving the barrel, because every minute fraction of a second that elapses provides more time in which the gun can move off the point of aim, and the bullet's path is determined by the position of the muzzle when the bullet leaves it. Therefore, super accurate rifles combine extremely short lock times (time lag betweeb sear release and cartridge firing), and barrels short enough to allow the bullet to exit quickly.

There is no "perfect" barrel length. While you could certainly determine the barrel length that results in the maximum velocity for any given cartridge, that barrel may be so long that it is far from perfect. It would be heavy, awkward, prone to vibrations and warping, etc. As with most things in this world, optimal barrel length is a function of balancing competing factors. For most centerfire cartridges, the optimal length will be between 14 and 28 inches, depending on the specific cartridge and the purpose of the particular rifle. Of course, that makes sense because 16"-24" seems to make for a fairly "handy" rifle, and modern cartridges have been designed to use projectiles and propellants that work best with barrels in that range.

 

[Harley Quinn]

Those who shoot 1000 yd match, are into long barrels I believe

Lots of information on 223 but not barrel length.
http://mywebpages.comcast.net/jesse99/rifle223palma.html

 

[JohnBT]

"Rimfire target matches are typically won by rifles with short barrels, because they are more accurate than longer barrels."

Short barrels are more accurate? You mean short like 16 inches? I guess I've been reading benchrest.com too much and hanging out at K.C. Young's range too much.

For example, from the current page one at benchrest.com

http://benchrest.com/forums/showthread.php?t=46800

There's a question about Benchmark barrels. Kent Owens' reply: "I have a light reverse taper on one opf my 40X's and it's .750 at the smallest spot and goes to .850 at the muzzle, 24" and it's fitted at 24" length."

Joe Friedrich's reply: "The reverse taper that is on my heavy rifle is .750 at its smallest and .915 at the crown. the barrel is 25 1/4 inches long, if i take the 1 and 1/4 inches off to make it 24 inches long like Kents it would be .908."

A search will turn up more.

John

P.S. - A little on Mr. Friedrich www.6mmbr.com/gunweek078.html

"Joe was the ARA Aggregate Champion for 2006. On March 25, 2007 he shot a perfect 2500 score--only the sixth man to have accomplished that feat at the time. In the same match, he recorded an amazing 9850 total 4-target score (2450, 2500, 2450, 2450). By all accounts, this is the best 4-target ARA Agg ever shot, by far."

 

[Father Knows Best]

Benchrest is different, because the rifle is rested. Therefore, the potential for movement of the rifle between sear release and bullet departure is almost nil. In competitions like rimfire silhouette where you must shoot from standing and other positions without a rest, shorter barrels provide a significant advantage for the reasons I explained above.

 

[trstafford]

Powder Type

Check out a reloading manual. They typically start with faster burning rate powders and progress up to slower rates. The faster burning powders build up pressure much faster but tend to make less muzzle flash. However large capacity cartridges can get into real problems trying to use fast burning powder.

 

[elmerfudd]

At a certain point the length of a barrel starts to slow down a bullet, but it's not because the gases behind it have stopped pushing it, but rather because the column of air that the bullet has to push out of it's way gets longer as you increase barrel length. Basically you add the mass of the air in the barrel to the mass of your bullet.

I once read an interesting experiment where they used 10 foot long chunks of barrel chambered in .32 ACP to try to determine ideal barrel length. It took something like 30' of barrel before the bullet would come to a stop, but if they made the barrel longer than 30' then the bullet would stop sooner. The extra weight of the air that the bullet was having to push out of it's way slowed it down more. Interestingly, they didn't find friction to be particularly important and there appeared to be a large span where the bullet reached and retained it's near optimal velocity.

I'm not really sure that this is of much importance, because the barrel lengths in question are absurdly long, but I definitely found it interesting.

 

[rangerruck]

there is a volume measurement for this, but it is not practical. what has to be done is a numerical value placed upon rate of expansion and a few other things.
I don't remember all the particulars, but the guntester on the article , and the guys from Hornady, wer doing some independent testing, and they came up with a expansion number ideal for rimfires of 19. now then, this could be achieved in a shorter bbl for 22's, about 15 to 17 inches, for mach 2, it was about 19 to 21 inches, and for 17hmr , it was about 22 to 24 inches. These measurements gave them the most ideal, consistent chronied numbers, with no measureable "blips" in velocity, when fed into a computer program, to determine other data and statistics.
now for centerfires, this value number was much higher, 29 or 39, I don't remember, but it is not practical to reach this number in most centerfires, as you would need a longish bbl. usually 30 inches or more. And to capture max velocity period, if that is your goal, then for all the gained speed from expanding gas, you would need something like a 5 ft. bbl or more , to maximize velocity gain. So this number is really not necessary, it is better to get near or on the expansion ratio number.

 

[JohnBT]

"Rimfire target matches are typically won by rifles with short barrels, because they are more accurate than longer barrels."

"Benchrest is different"

First you say short barrels are more accurate and then you say benchrest doesn't count as a target match. I stick by my statements. If short barrels were more accurate then the winning rimfire benchrest shooters would all be using them.

We could even go back and look at earlier rimfire rifles like the Winchester Model 52 and Remington Model 37 that won so many matches. Not a short barrel in the bunch. Or even production rifles like the Suhl 150.

The Anshutz 1827 Fortner Biathlon rifle has a 21.6" barrel and there is likely no target match as physically demanding as a ski race that requires you to stop and shoot while your heart is pounding and your lungs are burning.

John

 

[Father Knows Best]

If you read my posts again, you will see that my point was that there is no inherently "perfect" barrel length. It's a matter of compromise. Shorter barrels have some advantages, and longer barrels have other advantages. It's a matter of finding the optimal balance for the particular cartridge and shooting event. My point, however, was that in any event where movement of the firearm is a significant concern, then shorter barrels offer a significant advantage because they allow the bullet to leave the barrel more quickly, which minimizes the effect of the movement of the gun. That is particularly true with slow moving projectiles, such as you have with rimfire competitions. In events like benchrest centerfire, the potential for rifle movement is far less, and the bullet is moving much more quickly, hence the advantage gained from the shorter barrel is very slight and may well be offset by the disadvantage that comes from the lower velocity. Longer barrels provide higher velocity, which decreases time of flight to target, and minimizes the effect of wind. Therefore, benchrest rifles tend to favor longer barrels than rimfire silhouette rifles.

I'm not a biathlon expert, but I'm confident that the barrel lengths used were arrived at by a lot of trial and error to find the best trade-off. If you're not allowed to use optical sights (which you're not in biathlon, if I'm not mistaken), and you can't use a barrel extension to get your front sight out beyond the muzzle, then a significant disadvantage of a short barrel is the reduced sight radius it provides. Of course, just as a 48" barrel would provide a great sight radius but be difficult to shoot, a 24" barrel likely has more disadvantages in a biathlon rifle than the 21.6" barrel on the Anschutz you mentioned.

The bottom line is that longer barrels are NOT inherently more accurate than shorter barrels. It depends entirely on the situation -- sport, cartridge, sights, etc. All else being equal, however, shorter barrels are stiffer and allow the bullet to exit more quickly, and those things are advantages to accuracy. Whether they are offset by shorter sight radius, lower bullet velocity, etc., depends on the situation.

 

[aspade]

At a certain point the length of a barrel starts to slow down a bullet, but it's not because the gases behind it have stopped pushing it, but rather because the column of air that the bullet has to push out of it's way gets longer as you increase barrel length. Basically you add the mass of the air in the barrel to the mass of your bullet.

Energy spent pushing the column of air out of the barrel is trivial.

A 24", .30 caliber barrel has about 22" of air space ahead of the bullet. The volume of this cylinder, dimensions 0.305 x 22, is about 1.6 cubic inches.

1.6 cubic inches of air weighs about 0.3 grains.

The bullet will stop accelerating when friction with the barrel (which increases with speed) becomes greater than the push of powder gasses on its base (which decreases as the bullet travels down the barrel and the effective chamber becomes larger).