TargetTalk

These may sound like a silly questions... but are they? Let's have a thoughtful discussion. My apologies if this has been done to death before in this forum.

Why do guns designed for accuracy characteristically have long(er) barrels?

Consider these:-

A century or so ago, FW Mann showed that consistency of entering the projectile into the barrel, such that its centreline was on the bore axis, and that its centre of gravity was also on the bore axis was critical for accuracy.

He also showed how important the condition of the muzzle was for accuracy.

He also showed how very short barrels cause lead projectiles to be upset and deformed.

Given that the 'barrel time' of the projectile will tend to be proportional to barrel length, and that muzzle deflection, due to recoil, will start as soon as the projectile begins acceleration, muzzle deflection might be greater with longer barrels. (I'm know that, because of the projectile's acceleration and pressure - time characteristics, barrel time is not directly proportinal to barrel length. Also, longer barrels will have a greater moment of inertia, making deflection somewhat less, all other things being equal).

Barrel vibration effects might be thought to be more of a problem with longer barrels.

So my question perhaps should be - what does the bit of barrel between breech / lead and muzzle contribute to accuracy?

Next question:-

Why are bullets of a shape that is aerodynamically unstable, such that they have to be stabilized gyroscopically (with all the problems that causes)?

Putting aside the velocity gains and powder burning stuff, the bit of barrel between the leade and the muzzle moves the front sight away from the shooter, resulting in a longer sight radius, making it easier to aim the rifle. Benchrest shooters use shorter barrels than the metallic sight target shooters do, but they always use scopes.

From a practical standpoint, the bullet needs to be heavy in proportion to its size, i.e. dense, in order to be effective on the enemy or game, so gyroscopic stability is the way to go. Aerodynamic stabilization would require much longer projectiles to keep all that center-of-gravity/center-of-pressure stuff in order. What problems with gyroscopic stabilization are you referring to?

One school of thought is that 20+inches is sufficient for max accuacy,with of course a bloop tube to give sufficient sight radius.This also has the effect of lightening the rifle for those looking for a little less weight.
Anschutz went this way some time ago with success, not sure if they still offer this option.[/quote]

In benchrest, 24 - 25 inch barrels are common with length of barrel maybe getting longer in the last few years, 27 and 28 inches. We are going to longer skinnier barrels with a tuner attached to the muzzle for tuning barrel vibrations. A skinny long barrel is easier to tune than a short fat one. Without a tuner though this barrel is not that accurate. Barrel time is not a concern in benchrest and some think the longer barrel gives a more consistent velocity.

In 3P the short barrel with a bloop tube gives the advantage of less barrel time. Even some prone shooters are going this route as less weight gives them more endurance in the long matches. A few are using tuners incorporated into the bloop tube with some success.

Beyond about 18 inches the .22 slows down in the barrel. Not much, just a tad. The .22 at just below the speed of sound reacts the opposite of centerfire ammo. Centerfire deflects less in the wind the faster it goes. The .22 has less wind deflection at a slower speed. Example, standard velocity deflects about 1.0 inches in a 10 mph crosswind at 50 yds. Hi-velocity deflects about 1.1 inches in the same condition.

or is wind deflection more related to 'loss of velocity' than to velocity?
Spencer

Thank you 'Guest', your observations tend to confirm my from-first-principles suspicions.

I'm disappointed that there has been so little response. My suspicion is that the barrel between lead and muzzle really isn't that important (bent and bulged barrels can still shoot remarkably consistently) - it just separates the two important bits. But what is the optimum separation, and what physical processes are responsible for the optimum length? I understand the importance of adequate sight separation, but this can be independent of barrel length.

Harold R Vaughn's computer simulation work, some time ago, showed that a heavy, flexible barrel was desirable for accuracy. I don't think he looked in detail at damping. What you say about bench-rest barrels being relatively long, and damped, is therefore interesting.

Is it really true that higher-velocity .22 projectiles are deflected more by cross-winds than slower ones? Now why is that? Is it perhaps differences in rotational speed, altering the projectile's precession / coning effects rather than forward speed which is important?

Why do guns designed for accuracy characteristically have long(er) barrels?

They don't have longer barrels than needed to achieve the optimum bullet stabilization and velocity, look at the short airrifle barrels used in 10 meter competition and the short 100 and 200 yards benchrest barrels.
For smallbore the barrels are even so long that they reduce the bullet's velocity in the end part of the barrel which is beneficial to accuracy for the smallbore bullets because they are more stable at a subsonic velocity. (the speed of sound differs with airdensity, which differs with altitude, pressure, humidity and temperature.)

So barrel lenght is a result of obtaining the optimum projectile stability and speed for the chosen projectile and shooting distance, keeping in mind that for bucking the wind and reaching longer distances with some accuracy you need to achieve higher velocity (actually you need shorter time of flight if you want to reduce wind effects) in supersonic projectiles for which you need a longer barrel to burn more powder resulting in higher thrust before the projectile exits the muzzle

Furthermore it is probably beneficial to accuracy if the gas jet leaving the barrel behind the projectile is more consistent from shot to shot and under different circumstances, so it exerts its force on the projectile in the same manner every time. A longer barrel will average differences in pressure and shock wave of the gas jet. This goes for airrifles as well as for firearms. These gas jets have a rather big effect on the projectile for the first inches after the muzzle (read Harold Vaughn's book on this for more info.)

Anschutz does not offer the short barrels with sight extension tube anymore. I've never heard why from Anschutz themselves but opinions i've heard say that it is because they more often show changes in accuracy / more erratic accuracy than longer barrels. And that this is due to 1. the fouling of the extension tube (and average shooters not cleaning those frequently enough) 2.higher bullet velocities with associated lower stability in windy circumstances3. higher gas jet forces on the bullets 4. the barrel vibrations being less optimal.

Smallbore bullets are really more stable (especially in the wind) at lower velocities. This has been tested. I thought I read a test about this at the delphi smallbore journal. There certainly is an optimum between time of flight and stability when looking at the effect of side wind. I think i read that a good optimum was at 1000 fps when taking in account the differences in the speed of sound at different altitudes, temps etc.

I hope i've added some usefull arguments to give an answer to why we use longer barrels for more accuracy. Let the discussion go on......

Sven


[/quote]

The following on wind deflection is copied from the NRA Fact Book:

Those unfamiliar with smallbore rifle competition are often surprised to learn that target shooters almost universally use standard velocity ammunition in preference to the many varieties of high and hyper-velocity rounds available. The reason given is the low-velocity ammunitions resistance to wind deflection.

Despite what might seem at first to be the case, wind deflection is not proportional to the time of flight. Instead, it is proportional to the amount of delay in the flight caused by air resistance. The 1145 f.p.s. standard velocity .22 long rifle round takes .287 seconds to go 100 yds., but would take only .262 seconds to cover the same distance in a vacuum. The latter figure is easily found by dividing 300 ft. by the speed of the bullet (1145 f.p.s.), which would remain the same throughout its flight if it were in a vacuum. Thus the delay caused by air resistance is .025 second with the standard velocity ammunition.

The 1335 f.p.s. high velocity ammunition, which will take .259 second to cover 100 yds., would take only .225 second in a vacuum. Thus, the delay for this bullet is .035 second or 37% greater than that of the standard velocity round .22.

The high speed round, then, suffers about 37% more wind deflection than the standard velocity.

This remarkable result is due to the very rapid rate at which air resistance increases with increase in bullet speed in the region near the speed of sound. The .22 rimfires are the only important rifle cartridges that occupy this speed range, and they are the only ones that show more wind deflection as velocity is increased.

While air resistance always increases when the bullet is shot faster, the rate of this increase is less steep at supersonic velocities. Rifle bullets in general, contrary to the case of rimfires, are made less sensitive to the wind by raising their velocities.

NRA Firearms Fact Book

Astounding, but to take this to rediculous lengths -- would not a bullet going very slowly (with very low drag) not be the least affected by wind. I tend to think this is not the case.

jrmcdaniel wrote:Astounding, but to take this to rediculous lengths -- would not a bullet going very slowly (with very low drag) not be the least affected by wind. I tend to think this is not the case.

Undoubtedly, it's not. As the article alludes, there's a sharp rise in turbulence near the speed of sound. Outside that region, either above or below, flight time would matter more.

I quickly read the chapter in Harold Vaughn's book about the effect of wind on (supersonic) bullets this morning during breakfast.
He writes that wind deflection is caused by the nose of the bullet lining up with the directon of the wind. So the nose is pushed of the 90 degree angle it makes with its line of flight when there is no wind. (or in other words there is an angle greater than zero between the centerline of the bullet and the line of flight, it has a yaw). So the bullet is not pushed aside by the wind but it points its nose into the wind. This changes the horizontal trajectory because the drag is different on the left and right sides of the bullet causing the bullet to move to the right or left side depending the wind direction.

The amount of deflection from the zero angle depends on the gyroscopic stability of the bullet (the wind force and direction being the same).

Wind deflection not only causes horizontal displacement of the bullet but also (a much smaller, about a fourth) vertical displacement. In barrels with clockwise turning grooves the vertical displacement is down when the wind comes form the left side and up when the wind comes form the right side. This is the effect of the wind on the gyroscopic stability of the bullet that is spinning clockwise. (in counterclockwise grooved barrels the vertical displacement is in the other direction)
This shows that the effect of wind on bullets is caused by the changing nose angle (yaw) of the bullet and not by the wind pushing the bullet aside.

So a less gyroscopic stable bullet wil have more wind deflection.
I did not read anything about the effect of drag on wind deflection. But a bullet with more drag will slow down faster and have a lower speed at the same point in flight. And I guess a lower speed will result in a lower gyroscopic stability and this explains the effect of drag on wind deflection.

So the effect of wind is caused by the effect on the gyroscopic stability and not in the first place by time of flight as I stated earlier.

This explains clearly why a slower but more stable smallbore bullet reacts less to wind than a faster but less stable bullet. The loss in stability being caused by the bullet getting closer to or breaking the sound barrier.

Of course wind deflection is not everything. Match ammo is a compromise. Most is around the 1050 range. Too slow looses accuracy and to fast to much wind deflection. When Federal was making their great ammo a few years ago, they first came out with 1000A. It was the ultimate in accuracy maybe. But complains arose over its wind deflection so they came out with the 1000B, a tad slower. Both were used in benchrest, but 1000B was a little more popular. Still Lapua Midas was more popular during that time. I tried some lots of the Federal and it bit me in another way, leading. I was not the only one. That is probably the reason many did not shoot it was it leaded so bad in some guns you couldn't get through a target without accuracy falling off. And hard to clean. But we have an assortment of better cleaning products today that weren't there in the late ninties. So much to consider when looking at rimfire performance.
Champions Choice has the short barrel 2013/500 in the aluminum stock in their 2007 catalog as well as the longer barrel 2013/690.