Entre 20 et 24 pouces=70FPS de différence sur ce graphique.
D'autres comparaisons faits avec des armes de haute précision.
http://www.tacticaloperations.com/SWATbarrel/
Article photos by Stan Nielsen Copyright © 2000
Eugene Nielsen
BARREL LENGTH AND THE PRECISION RIFLE
Why shorter barrels may often be better
by
Eugene Nielsen
There’s a growing trend to shorter barrels on tactical precision rifles. In years past, a 24- to 26-inch barrel was practically a given. Accepted wisdom was that it was necessary to sacrifice a little maneuverability to gain a more complete powder burn and significantly reduced flash signature. Today, it’s not uncommon to rifles with significlly shorter barrrels.
Attitudes are are changing. The desire for more manueverable rifles for the urban setting has led a growing number of manufacturer's to come out with shorter-barreled precision rifles. This brings up an obvious question -- how short is too short? What sacrifices, if any, are made by going to a shorter barrel?
To answer these questions, we must first start by taking a look at the subject of internal ballistics. Internal ballistics is a very complex subject. There are many factors which affect the internal performance of a given cartridge and bullet. Factors affecting internal performance include the powder chamber capacity; load density; amount and burning characteristics of the propellant powder; temperature of the propellant prior to ignition; uniformity and speed of ignition; diameter, weight and bearing length of the bullet; and the length of the barrel and its interior dimensions.
Longer barrels give the powder more time to work on propelling the bullet. For this reason longer barrels generally provide higher velocities, everything else being equal. However, the gas pressure behind the bullet diminishes as the bullet moves down the bore. Given a long enough barrel, there will eventually be a point in which the bore friction and air pressure in front of the bullet will equal the gas pressure behind it. At this point, the velocity of the bullet will start to decrease.
Shorter barrels are capable of surprising long-range accurancy. San Fernando (California) PD Special Response Team long rifle marksman Chris Colelli, shown here, fired a three-shot group at 700 yards that measured just under two inches center-to-center. Colleli also fired a sub-1/4- inch group at 200 yards. The target is now framed and hanging in his lieutenant's office.
There isn't any clear-cut answer as to how much velocity will be lost per inch of barrel length reduction. The amount of loss is closely tied to the expansion ratio. As previously noted, the type and amount of powder, as well as the weight and bearing length of the bullet, also play a major part. Rifles with high expansion ratios (smaller calibers) tend to lose less velocity than rifles with low expansion ratios (larger calibers).
Tactical Operations about in the April 2000 issue of S.W.A.T., typifies the trend to rifles with shorter barrels. Tac Ops considers a barrel of length of 18 to 20 inches to be optimal for the urban environment, with 18 inches the preferred length.
During the development of the Tango 51, Tac Ops took a standard 26-inch barrel and cut it down to 18 inches in one-inch increments. Between 10 to 20 rounds were fired at each invrement. They found that a 20-inch barrel provides for a complete propellant burn and no velocity loss when using Federal Match 168-grain BTHP, a cartridge that has become something of a law enforcement standard. Going to an 18-inch barrel only resulted in a loss of 32 feet per second (fps).
Shorter barreled rifles are more versatile, being equally suitable for both urban and rural operations. According to Tac Ops, there isn't any need to go to the 26-inch barrel unless you want to go to a heavier bullet or push the round to higher velocity using more powder or use a slower burning powder. The Los Angeles County Sheriff's Department's Special Enforcement Bureau (SEB) performed tests similar to those conducted by Tac Ops and came to similar conclusions.
Tommy Lambrecht, SEB armorer and Special Weapons Team long rifle expert, recently chronographed the Federal Match 168-gr. BTHP rounds. Lambrecht said that the muzzle velocity was averaging around 2,660 to 2,670 feet per second (fps) from the 20-inch-barreled Tango 51 that Tac Ops delivered to him.
The accuracy of the Tango 51 isn't hampered by the shorter barrel. While at the range with the Tango 51 we were consistently getting sub-1/4 MOA accuracy at longer ranges? Well, the shorter barrel doesn't hamper longer range accuracy either.
As I mentioned in my article on the Tango 51, San Fernando (CA) PD Special Response Team long rifle marksman Chris Colelli once fired a 3-shot sub-1/4-inch group at the article appeared, Colelli fired a 3-shot group from the rifle at 700 yards that measured just under 2 inches center to center. The group, which was witnessed by several credible spotters, was shot off of a bipod with one small sandbag.
Los Angeles County Sheriff's Department Special Enforcement Bureau (SEB) Special Weapons Team long rifle experts Fred Keelin (left) with Tac Ops Alpha 66
Colelli is a superb marksman, one of the best that I've seen, but he would be the first to admit that an element of luck played a role in this feat. Groups like these certainly aren't typical of what could be realistically expected under actual operational conditions. Still, they show that the rifle is capable of phenomenal accuracy provided that the operator does his or her part.
Although the 20-inch barrel remains very popular with agencies purchasing the Tango 51, many agencies prefer an 18-inch barrel for its added maneuverability. With the 18-inch barrel, you're still shooting around 2,630 fps with Federal Match. The target certainly isn't going to know if he's being hit with a bullet that leaves the muzzle at 2,660 fps or 2,630 fps. The terminal ballistics are identical.
Going to an 18-inch barrel doesn't adversely effect the accuracy of the rifle. Tac Ops has achieved incredible accuracy with the shorter barrels. The 18-inch barreled Tango 51 rifles will still shoot sub-1/4 MOA. The performance is just as good with the 18-inch barrel as it is with the 20-inch barrel out to a distance of 600 yards. After initially going with the 20-inch barrel for their Tango 51s, the Los Angeles County Sheriff's Department has decided to go with the 18-inch barrel and Tac Ops 30 suppressor on all new Tango 51s that they purchase.
Shorter barrels are actually often more accurate than their longer counterparts. A rifle barrel is a cantilevered beam and as such they sag. More sag results in more whip and vibration as the bullet travels down the bore. Barrel sag induces longitudinal stress that can cause stringing of shots. Using a shorter, heavier barrel minimizes reduces stress and accuracy-robbing barrel vibration. A shorter barrel is stiffer and vibrates at a less.
Barrel length and contour determines the relative "stiffness" of a barrel, i.e., how much a barrel will tend to vibrate. Shorter barrels generally have oscillations of smaller amplitude. than longer barrels. Thicker barrels generally have fewer vibration nodes than slimmer barrels. The ringing frequency of a thicker barrel is higher and the oscillations are of a smaller amplitude and of a shorter duration. This equates to less barrel motion at the muzzle. The use of a shorter barrel also allows the use of a heavier contour without making the rifle unwieldy.
The use of a heavier contour tends to provide less variation between a cold shot and any subsequent follow-up shots. Barrels expand as they heat up. As the barrel expands any stress on or in the barrel will cause stringing of the shots. Bore expansion results in an increase in group size. Heavier barrels tend to be more consistent because they take longer to heat up.
An 18- to 20-inch barrel may be fine for a caliber like the .308 Win., but what about calibers such as the .300 Winchester Magnum (7.62x66B)? Many agencies are opting for this cartridge as a result of its long range ballistics. The .308 Win. has a maximum effective range of about 800 yards. While this is certainly more than enough for most law enforcement scenarios (law enforcement snipers rarely have to engage targets at more than 100 yards), the .300 Win. Mag. does increase the maximum effective range by an additional this comes with the price of additional recoil.
Many agencies purchasing a .300 Win. Mag. will primarily be employing the rifle in an urban environment. The common reason for opting for the .300 Win. Mag. that it extends the capabilities of the rifle to longer ranges than the .308 Winchester is capable in those rare situations where longer range capability is necessary. This leads to an obvious question -- will going to a shorter barrel for added maneuverability in the urban environment adversely affect long range performance of a rifle in this caliber?
To find the answers, Tac Ops took a 26-inch barreled .300 Win. Mag. and chopped the barrel down in one-inch increments as they previously did with the .308 Winchester. Ten rounds of Federal Match 190-grain BTHP Gold Medal were fired from each increment. No velocity was lost from 26 inches to 22 inches. Velocity loss started to occur only after they went below 22 inches.
As a result of their tests, Tac Ops decided not to go below 22 inches on their .300 Win. Mag. tactical precision rifle, the Alpha 66. According to Mike Rescigno, President of Tac Ops, the 22-inch barrel is ideal for the tactical shooters that are going to use the 190-grain Federal Match ammo. There isn't any loss of performance by going to the 22-inch barrel and this round. The Alpha 66 still provides 1/4-MOA or better accuracy.
For heavier bullets or hotter loads with slower burning powders, Rescigno recommends a 24- to 26-inch barrel. The longer barrel length is necessary for complete powder combustion with these loads. Rescigno adds that he has a 24-inch barrel on his personal .300 Win. Mag. just in case he wants "to shoot the heavier 220-grain bullets with a lot of powder."
TOP: After testing and evaluation, the SEB has decided to go with an 18-inch barrel on all new .308 Win. Tango 51s, and a 22-inch barrel on all .300 Win Mag. Tactical precision rifles. Fred Keeling (left) is shown with with a suppressed 18-inch-barreled Tango 51 .308 Win., and Guy Geisler (right) is shown with a 22-inch-barreled Alpha 66 .300 Win. Mag.
BOTTOM: The optional barrel length is closely tied to the loads that will be employed. There is no loss of performance from the .300 Win. Mag. Federal Match 190-grain BTHP when fired from barrels as short as 22 inches. The above Alpha 66 has Tac Ops muzzle brake installed.
At this point, I can hear readers asking, "What about muzzle blast and muzzle flash? Won't they be a problem with the shorter barrels?" These are valid concerns. With both calibers, shorter barrels do increase the muzzle blast and muzzle flash somewhat. It's not as much as one might expect. From a practical standpoint, the differences between a 24- or 26-inch barrel and an 18- or 20-inch barrel are negligible, except when slow burning powders are used.
Any concerns over the muzzle blast and sound/flash signature can easily be eliminated by the use of a sound suppressor (silencer). With today's compact, low-maintenance suppressors, such as the Tac Ops 30, there's no reason that all tactical precision rifles shouldn't be so equipped. More and more law enforcement agencies are coming to this conclusion.
The use of a sound suppressor provides a number of advantages to both the shooter and spotter. The suppressor greatly reduces any ground disturbance and eliminates any muzzle flash/sound signature that can identify the position or disturb vision and hearing. There isn't any necessity for the shooter or spotter to wear hearing protection. Many shooters find that their accuracy improves when a suppressor is employed due to the resulting reduction in the muzzle blast and recoil. The reduction in recoil also permits quicker follow-up shots.
A sound suppressor can substantially reduce the recoil velocity and recoil energy of a rifle. Gas volume and gas pressure at the muzzle are major factors in the free recoil energy produced by a rifle. Shorter barrels generally result in increased gas volumes and higher gas pressures at the muzzle. All other factors being equal, increased gas volumes and higher gas pressures at the muzzle will increase the recoil velocity and free recoil energy.
Free recoil energy is proportional to the square of the recoil velocity of the rifle. Doubling the recoil velocity quadruples the free recoil energy. Sound suppressors reduce the free recoil energy by suppressing the effects of the expanding powder gasses. They also add weight, slowing the acceleration of the rifle.
In summary, the appropriate barrel length is closely tied to the caliber and the load or loads that will be employed. If a shorter barrel provides equivalent or better accuracy and little or no loss in velocity, why go to a longer barrel? Why sacrifice maneuverability and add excess weight? While old attitudes may die hard, chronographs and ballistics don't lie. Shorter barrels are often better. The proof is in the performance.
SOURCE
Tactical Operations, Inc.
433 North Camden Dr. 4th Fl. #239
Beverly Hills, Ca 90210
Phone 310 275-8797
Fax 323 933-3521
Différentes poudres utilisées peuvent aussi influencer les données.
Plus un canon est long, plus il va amplifier la moindre erreur du tireur.
Accueil 
Ven 22 Avr 2016 - 13:24
qui sait ,mais la si le gars veut sans faute tirer sur papier et est anti -chasse oui effectivement il pourrait y aller en 20 pouces pour gagner ce 100 pi/ sec de plus. 