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Old February 14, 2013, 08:18 AM   #1
Kimio
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A little confused, questions about barrel length and velocity

Okay, so I'm a little confused here.

For the longest time, I was told that typically the longer the barrel the greater the velocity and amount of spin is imparted on the bullet. This of course varies from round to round due to bullet load and twist rate, but the general idea has remained the same, depending on the round, you will need x twist rate and x amount of barrel length in order to stablize the round for extended distances.

So lets take the .308 for example, I was under the impression the ideal length for san an FAL or an M1A would be at least 18" in length (17" and some change if you want to get specific) since you get the better part of both worlds, you have enough length to stablize the round for longer distances (300yd maybe?) while not having the barrel so long that it becomes cumbersome (I'm not humping the rifle up a mountain so it's not as big of a concern for me)

shorter barrels will cause a loss in velocity and therefore a loss in accuracy, at least in the case of the .308 cartridge.

Now I'm being told that the loss in velocity is neglible and that shorter barrels can actually increase over all accuracy due to changes in the barrels harmonics and rigidity (I though this was affected more by if the barrel is free floated and also depended on the type of materials used in its manufacturing)

So which is it? I keep getting mixed responses, part of the reason I'm asking is because I'm debating on building a 16" FAL PARA, the rifle probably will not be shot out any farther than 300yd 400yd if I'm feeling lucky, and while I'm not expecting to get 1MOA our of it I am hoping to at least get MOM (Minute of Man) out of the rifle with the shorter length barrel (I like the handiness the carbine version of the rifle)

Clarifications please?
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Old February 14, 2013, 08:42 AM   #2
Brian Pfleuger
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1) Barrel length has no direct influence on accuracy. With a particular load, yes, you could shorten the barrel to match the muzzle exit time of the load.... or you could work up a better load to match the existing barrel... which seems easier.

2)Barrel length has no direct influence on bullet stability. Longer barrels produce more velocity for a given load but what matters is the velocity and the twist rate. You might not be able to get the velocity to stabilize the bullet from a shorter barrel but it's not directly the barrel's length that matters. Also, the difference between an 18" and a 24" barrel could be as little as 150fps, possibly less. A bullet that's unstable at the 18" velocity is still not going to be much more stable 150fps faster.

3)Generally speaking, once a bullet is gyroscopically stabilized, it will remain so, for practical purposes. It will hit the ground or the target before it destabilizes. The resistance to it's forward movement is much greater than the resistance to it's spin. In other words, the spin doesn't slow down much and the spin is what makes it's stable, not the velocity.
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Old February 14, 2013, 09:01 AM   #3
Kimio
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Hmm, boy was I ever wrong then. Thank you for that explaination Brian.
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Old February 14, 2013, 10:00 AM   #4
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Quote:
shorter barrels will cause a loss in velocity and therefore a loss in accuracy, at least in the case of the .308 cartridge.
Brian is correct. Velocity isn't accuracy. Sometimes its a deterrent to accuracy.

You mentioned the 308. The Army learned a long time ago, the most accurate velocity for the 308 in International Rifle Shooting was right at 2200 fps.

This velocity was increased for gas guns (match ammo as in the M14) to 2550, not to increase accuracy but to work the action.

Longer barrels do offer a benefit in target rifles where the use of iron sights are used. Not because the added velocity is needed but because the longer barrel gives you a longer sight radius, and the longer sight radius, the more accurate you can shoot the rifle.

The Mann accuracy devise the Army uses to test 308 (7.62) ammo is a 1903a3 action with an 18 inch barrel.

Velocity also comes in when hunting. For example you max hunting range you expect to shoot a given critter needs a certain about of KE to make a humane kill.

My granddaughter did a science project a few years ago, (I have posted the results before) where she was showing the shortest barrel one could have on a 308 using a 180 Gr Remington Core Lock bullet to make it convenient handling yet keep KE to about 1800 ft lbs. for elk hunting.

She cut a 26 inch barrel two inches at a time to 16 inches checking the velocity as she made the cuts.

Accuracy wasn't affected.
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Old February 14, 2013, 02:06 PM   #5
Bart B.
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Kraig, your comment
Quote:
The Army learned a long time ago, the most accurate velocity for the 308 in International Rifle Shooting was right at 2200 fps.
is familiar with me. What bullet weight was that used for; Sierra's 168's?

When around 1970 the USN team had problems with one lot of Sierra's 168's coming apart in the first 100 yards fired from MexiMatched M118 ammo in 1:12 twist Garand barrels, we called Sierra to ask about that. Martin Hull, their ballistician told us the US Army Rifle Team also had the same problem in their M14's 1:12 twist using the same ammo with that lot number of 168-gr. bullets. And the AMU's Barnett 1:10 twist barrels made 'em blow up closer to the firing line. So the Army gave them to the International Team folks for their 300 meter free rifles with reduced loads, those bullets shot very well indeed and probably the most accurate lot they've had. Sierra said they would replace that lot for both services; the Army kept theirs in the Int'l unit, the Navy got replacement ones.

I thought the M14's ammo was designed before the .308 Win. ammo was, not the other way around as your post implied. And I was told by LCAAP in the late '60's they used 21" and 21.5" barrels in their Mann accuracy devices as stated in:

http://www.odcmp.org/1001/mann_inc.asp

The two Mann devices shown below bought from CMP sales have 21.5 inch barrels.





And while 2200 fps out of a 1:12 twist bullet will stabilize that 168 very well, heavier and longer ones have to be spun at a faster RPM. But lots of records were set and matches won with reduced loads in bolt guns at 200 and 300 yards with that bullet leaving in the 2200 to 2300 fps range from 1:12 or 1:11 twist .308 Win. barrels. 40 or 41 grains of IMR4064 under 168's was a popular reduced load.

Kimio, as long as the barrel whips and wiggles at the same frequency for every round fired, it doesn't matter; accuracy will prevail if the bullet leaves at the right place every time in the muzzle's whip arc. And that's typically just before the muzzle angle reaches the top and starts coming back down. Every free floated barrel whips and wiggles with the same exact fundamental and harmonic frequencys for its shape with every shot fired regardless of the load used. Stiffer ones at higher frequencies, whippier ones and lower frequencies.
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Old February 14, 2013, 09:55 PM   #6
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Brian's comment:
Quote:
Generally speaking, once a bullet is gyroscopically stabilized, it will remain so, for practical purposes. It will hit the ground or the target before it destabilizes. The resistance to it's forward movement is much greater than the resistance to it's spin. In other words, the spin doesn't slow down much and the spin is what makes it's stable, not the velocity.
I think tests showed bullets slow their rpm rate down about 10 to 15 percent over 1000 yards; much less than velocity drop off.

As long as the bullet remains supersonic, that's true When it goes transssonic, they become destabilized and move off the trailing trajectory axis and start to keyhole in targets. Example; M852 7.62 match ammo with Sierra 168's leaving the muzzle at about 2600 fps (2550 fps at 26 yards as spec'd) started doing this at about 800 yards in cooler tempertures.
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Old February 18, 2013, 01:04 AM   #7
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400 yards will be no trouble for a 308 with a 16-inch barrel, especially if you are concerned only with standard service rifle-level precision. When you want greater long range precision you can get yourself a rifle with longer barrel and a stock that's more amenable to it than the paratrooper stock. Your chosen rifle type determines precision more than a couple inches of barrel length.

As a general point, a longer barrel will give greater bullet speed, depending on choice of ammunition. For a given rifling rate, greater bullet speed will give faster spin and, for a given bullet type, it will give greater gyroscopic stability. Bullet spin rate does indeed depend on bullet speed.
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Old February 20, 2013, 11:58 AM   #8
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So longer is not better all the time .
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Old February 20, 2013, 10:24 PM   #9
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Quote:
So longer is not better all the time .
That's debatable

When using the .308, benchrest shooters prefer 30" and 32" barrels so that they can get a good enough velocity out of 155 gr bullets to be consistent at 1,000 yards.
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Old February 21, 2013, 07:38 AM   #10
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And those 32" barrels are probably getting less than 100 fps over a 22" barrel. So yes "longer is not better all the time". It is only better in very rare circumstances and even in this case you'd be much better off with another chambering rather than havng to resort to such extreme barrel lengths.

A barrel of 18-22" does about all 99% of the folks who use a 308 needs to do. Just my personal feelings, but I don't like to go under 20" and feel anythng over 22" is a waste in a 308. It is more about how the gun balances and handles as well as muzzle blast than bullet speed. I'd hunt any animal in North America with a 20" 308 and feel good about taking all but the big bears out to 400 yards. I'd limit shots on the huge stuff to under 100.
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Old February 24, 2013, 01:01 AM   #11
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Quote:
even in this case you'd be much better off with another chambering rather than havng to resort to such extreme barrel lengths.
Back in the day you couldn't do that in Palma!
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Old February 24, 2013, 10:59 AM   #12
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allaroundhunter, it wasn't the benchresters shooting .308's that had to use 30" barrels; they used 20 to 22 inch ones for benchrest some years ago.

It was the Palma rifles used in prone matches at 800 to 1000 yards with 147 grain arsenal bullets from 7.62 NATO ammo in 30" barrels (the Brits were the first to do so). Was the only way arsenal ammo could shoot those light weight bullets fast enough (3000 fps or so) to keep 'em supersonic to 1000 yards.

Jimro, when Sierra's 155 gr. 30 caliber bullets were first used in competition, some folks' 26 inch barrels would not shoot them fast enough to stay supersonic at 1000 yards; even at 6600 ft. altitude. In my own tests with two .308 Win. barrels with .3075"/.2980" groove/bore, diameters, the 32" one shot the same load out at a bit over 3100 fps; about 200 fps faster than the 26" one. 30" barrels are pretty much the world standard for Palma rifles using 147 to 155-gr. bullets in a .308 Win. case. A 25 to 30 fps change in velocity for each inch of barrel length's about normal for the .308.
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Old February 24, 2013, 03:06 PM   #13
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stiffness

shorter barrels, of the same profile, are stiffer. stiffer barrels osilate less.
less osilation means less verticle stringing. Longer barrels add velocity until the friction of the bullet begins to slow its travel. This does not occur in any lenght you would carry. For your purpose get the lenght you want to carry.
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Old February 24, 2013, 06:03 PM   #14
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bcarver, I disagree with your comment that less osilation means less verticle stringing. Long, whippier, flimsier barrels have oft times got less vertical shot stringing than shorter, stiffer ones.

Especially if slower bullets leave later in the barrel's whip than faster ones and the velocity spread is large. The more bullet drop downrange from the slower ones is compensated for when they all leave on the barrel's upswing. That was proved over a century ago with the British .303 rifles and it still holds true today.

It all depends on where the bullets leave in the muzzle axis as the barrel oscillates (whips) vertically.
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Old February 24, 2013, 06:52 PM   #15
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This is really interesting and a good education for this noncompetitive shooter, but I have some food for thought, or disputation. Suppose a 30-cal bullet leaves a barrel with 1-12 twist at 2000 fps. As it leaves, it is making one revolution in one foot of travel, because that is the twist condition that it is leaving. Thus, it will make 2000 revolutions in traveling 2000 feet, neglecting the small decrease in angular rotation due to drag. It will make only 300 revolutions in reaching a target at 100 yards. Suppose accuracy is not good, so you increase the velocity to 2200 fps. The 10% increase in velocity also produces a 10% increase in spin rate, but the bullet still makes only 300 revolutions in reaching a 100-yd target. If accuracy has improved, which it well might, is it the result of increased spin rate, improved harmonics for the more energetic load, or both? Or something else?
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Old February 26, 2013, 03:26 PM   #16
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To McShooty

It is likely a combination of all. You didn't mention the bullet weight. A 1:12 will stabilize up to a 168 pretty well. Anything heavier you might want a 1:10 twist. The increase in accuracy might just be the combination of harmonics, velocity, twist, bullet weight, etc that your rifle "likes", due to it's specific combination of these attributes.

This is why reloaders usually can increase the accuracy of a firearm by custom recipes in their reloaded cartridges. They can tweek any given cartridge to the point that they can find that combination that the rifle likes to shoot. I don't think anyone has figured out the algorithm needed to match a specific cartridge to a rifle configuration so you just start at 10% or so below max and add a grain or two at a time until it prints the smallest group.
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Old February 27, 2013, 11:43 AM   #17
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Not to be taken as gospel, and it depends on a lot of factors, but a fair rule of thumb is that you lose (or gain) about 50 fps for every inch of barrel. So a given load from a 26" barrel will have a bullet velocity about 200 FPS greater than one from a 22" barrel.

Sometimes people read that for a given rifle the "powder is all burned within x inches, so the bullet slows down after that." Certainly there is a point where greater barrel length will cause a bullet to slow down and eventually stop, but that point is well beyond any reasonable barrel length in a realistic rifle. But the assumption that when the powder is consumed the pressure drops to nothing is false (if that were true, the bullet would never exit). Even after the powder is burned up, the hot gas keeps expanding and pushing the bullet and that will continue until the bullet exits or, in our hypothetical lengthy barrel, stops as the gas cools and stops expanding.

Jim
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Old February 27, 2013, 12:30 PM   #18
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Quote:
Kimio I'm asking is because I'm debating on building a 16" FAL PARA, the rifle probably will not be shot out any farther than 300yd 400yd
A 16" barreled FAL should serve you well. I have an M14 with a 4 groove 16.25" barrel with a 1:10 ROT. It's set up for a sound suppressor and is more accurate than I am. When compared to a 22" M14 barrel the reduction in MV from the 16.25" is nominal and will probably come into play only if I tried to shoot tight groups beyond 600 or so yards. I'm not trying to shoot tight groups with this rifle so it's of no real concern to me.
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Old February 27, 2013, 12:56 PM   #19
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Quote:
So longer is not better all the time .
Velocity is one thing, accuracy another but for most things I've found the exact opposite to be true. Give me short barrels when ever possible.
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Old February 27, 2013, 01:09 PM   #20
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Rifling twists for the .308 Win barrel. . . . . .
1:12 twists in 24" barrels have given match winning and record setting accuracy at ranges up to 1000 yards with bullets from 172 to 190 grains. 1:11 twist barrels were "the one to use" for 168's with reduced loads up to 190's with max loads when the .308 was the only thing used in most high power competitions. And the rate bullet rpm's slows down is about 10% the rate their velocity does; they don't always spin 180,000 rpm's (3000 fps from a 1:12 twist) all the way to 1000 yards.

Velocity changes per inch of barrel length. . . . . .
Too many tests by folks and SAAMI have shown that the .308 Win. gets about 25 inches per inch of barrel length from 20 to 30 inches. Shorter, they're slower; longer, they're faster. That's using the same barrel starting out long then cutting off an inch for each series of test shots.

Best spin rate for bullets. . . . .
Whatever combination of muzzle velocity and twist that spins them at the right rpm to stabilize them all the way to the target is right. A 180-gr. bullet from a .300 Win. Mag's 1:13 twist barrel will shoot very accurate. That same bullet from a .30-06 or .308 Win. will do well in a 1:12 twist. From a .300 Savage, it'll need a 1:11 or 1:10 twist. Spinning 'em just fast enough to stabilize them is what short range benchresters do. Spinning them in the middle of their best rpm rate is best for longer ranges. Too fast a twist will cause those more unbalanced ones to shoot less accurate; they'll jump too far off the bore axis when they leave it due to cetrifugal forces.
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Old February 28, 2013, 11:11 AM   #21
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Rifling twists for the .308 Win barrel. . . . . .
1:12 twists in 24" barrels have given match winning and record setting accuracy at ranges up to 1000 yards with bullets from 172 to 190 grains. 1:11 twist barrels were "the one to use" for 168's with reduced loads up to 190's with max loads when the .308 was the only thing used in most high power competitions. And the rate bullet rpm's slows down is about 10% the rate their velocity does; they don't always spin 180,000 rpm's (3000 fps from a 1:12 twist) all the way to 1000 yards.
The 1:10 was used in the 30-06 from 150 grain to 220 grain. So why not the 1:10 for 308?
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Old February 28, 2013, 03:52 PM   #22
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Going to extremes, a pencil profile 7" barrel will be more consistent (accurate) then a 24" pencil profile. The shorter the barrel the stiffer, and more accurate it is.

The longer the barrel the more time it has to accelerate so the more velocity it has at the muzzle. In longer distances the faster the projectile travels the less time environmental factors have to effect it. IE wind, gravity, corelais etc. So the more consistent it becomes.

If you are shooting less than 200 yards, hey my 7" ar pistol does just fine. If you are going longer you want more velocity.
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Old March 1, 2013, 12:00 PM   #23
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Hand loading changes the variables. If you load a fast burning powder you can sometimes loose velocity past a given barrel length. As an example compare a standard 22 lr vs a CCI stinger. The standard lr will often loose velocity much past 18 in and the stinger with slightly more slower burning powder will gain velocity all the way to 28 in.
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Old March 1, 2013, 12:39 PM   #24
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very interesting thread.
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Old March 1, 2013, 06:27 PM   #25
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Joel Pendergraft set New IBS 1000-Yard Heavy Gun World Record with ten shots within 3.048” at 1000 yards with this monster gun.

It is water cooled and the barrel is very thick. Based on web pictures of unlimited class benchrest rifles, thick barrels are setting the trend.

Thicker, shorter, means stiffer.

http://bulletin.accurateshooter.com/...-world-record/







Here is another

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