Re: Ballistics out of a 16.5" tube
This is part of an article written by Chuck Hawks.
Barrel length, accuracy and ballistics
It is worth mentioning that a longer barrel is not inherently more accurate than a short barrel. Intrinsic accuracy is a matter of quality, not length. However, a longer barrel is generally better in terms of practical accuracy, because a longer and therefore heavier barrel (within reason) is easier to hold relatively steady from unsupported positions; thus it is easier to shoot a long barreled rifle accurately.
The length of the rifle barrel has a direct influence on the velocity obtained from the cartridge for which it is chambered. Ballistically, for centerfire calibers, longer is usually better. However, for carrying, handling and maneuvering in close quarters (like thick brush or getting in and out of vehicles), shorter is usually better. Some sort of compromise must therefore be reached.
Very long 27-30 inch barrels are seldom seen these days on repeating hunting rifles, although they are still occasionally found on single shot hunting rifles and target rifles. The longest barrels usually seen on hunting rifles today are 26 inches in length.
26 inch barrels are usually found on rifles chambered for high velocity magnum cartridges. A long barrel is required to burn the large amounts of slow burning powder used in this type of cartridge. Unfortunately, most repeating rifles with 26 inch barrels balance too far forward; they are muzzle heavy and slow to swing. The long barrel seems to hang up on every limb and outcropping of rock in the area and a hunting rifle so equipped can be very awkward carry in steep terrain.
For this reason, many magnum rifles now come with 24 inch barrels, which sacrifice some of the magnum's velocity. 24 inches is about the minimum barrel length practical for most magnum cartridges. Cut a magnum's barrel down to 22 inches and the muzzle blast and flash become intimidating. Also, magnum cartridges such as the .264 Win. or 7mm Rem. lose so much velocity in a 22 inch barrel that they show little ballistic advantage over standard calibers like the .270 or .280.
The typical barrel length for a repeating hunting rifle chambered for high intensity cartridges, such as the .243, .270, .308, or .30-06, is 22-24 inches. These are useful all-around barrel lengths for such cartridges. The highest velocity standard cartridges (.243, .25-06, .270 Win.), which achieve muzzle velocities around 3000 fps, are at their best in a 24 inch barrel and 24 inches is the SAAMI standard for almost all American centerfire rifle calibers. However, for cartridges such as the .257 Roberts, 6.5x55, 7mm-08, .308 Win., .30-06, .338 Federal, .35 Whelen and .350 Rem. Mag., which typically operate at 2500-2800 fps, the velocity loss in a 22 inch barrel is not extreme and a rifle with a barrel of this length usually balances and swings well.
Cartridges with smaller cases that operate at lower velocity, such as all .22 Rimfire cartridges, the .30-30, .32 Special, .35 Rem. and similar numbers, do well in 20-22 inch barrels. The very popular carbine versions of classic lever action rifles like the Winchester 94 and Marlin 336 usually come with 20 inch barrels. These short rifles ride well in a saddle scabbard, are easy to carry in rugged terrain and handle very fast in close quarters. Because they are light rifles, they balance well with a 20 inch barrel. Muzzle blast from these cartridges in a 20 inch barrel is less severe than from the larger high intensity cartridges in a 22 inch barrel.
Combine a high intensity cartridge with a 20 inch barrel, however, and the velocity drops noticeably while the blast becomes annoying. Still, a rifle with a 20 inch barrel, chambered for short action cartridges like the 7mm-08, .308 and .358 Win., makes a very effective mountain or woods rifle. These cartridges retain enough velocity in a 20 inch barrel for medium range shooting and the stubby barrel is less likely than longer tubes to get hung up on branches, overhanging ledges, rocky outcroppings and so forth.
To my mind it is hard to justify barrels shorter than 20 inches for any purpose. I have owned rifles with 18-19 inch barrels and in every case I wished that they had come with at least a 20 inch barrel. Very short barrels of standard contour (not bull barrels) tend to make the rifle muzzle light and unsteady to hold and swing. I like a rifle to balance between my hands, not toward the butt. Even .22 rimfire rifles balance better with 20-22 inch barrels, although in this instance the longer barrel has no ballistic advantage, since the .22 LR cartridge burns all of of its powder in about 16 inches. Very short barrels also increase the muzzle blast from high intensity cartridges to very annoying levels and the velocity loss is excessive.
Velocity loss (or gain)
It is worth noting that the velocity figures published in ammunition brochures and reloading manuals are sometimes taken in barrels different in length from those supplied on many rifles. I have seen various estimates of how much velocity is lost (or gained) when a barrel is not the same length as the test barrel in which a cartridge was chronographed. Here are some of them.
The 2001 Edition of the Shooter's Bible states, in the introduction to the Centerfire Rifle Ballistics section, "Barrel length affects velocity, and at various rates depending on the load. As a rule, figure 50 fps per inch of barrel, plus or minus, if your barrel is longer or shorter than 22 inches." However, they do not say what category of load to which this 50 fps average pertains.
Jack O'Connor wrote in The Rifle Book that, "The barrel shorter than standard has a velocity loss which averages about 25 foot-seconds for every inch cut off the barrel. Likewise, there is a velocity gain with a longer barrel." He went on to illustrate this using a .30-06 rifle shooting 180 grain bullets as an example, so his estimate was obviously for rifles in that general performance class.
Other authorities have tried to take into account the different velocity ranges within which modern cartridges operate. The Remington Catalog 2003 includes a "Centerfire Rifle Velocity Vs. Barrel Length" table that shows the following velocity changes for barrels shorter or longer than the test barrel length:
MV 2000-2500 fps, the approximate change in MV per 1" change in barrel length is 10 fps.
MV 2500-3000 fps, the approximate change in MV per 1" change in barrel length is 20 fps.
MV 3000-3500 fps, the approximate change in MV per 1" change in barrel length is 30 fps.
MV 3500-4000 fps, the approximate change in MV per 1" change in barrel length is 40 fps.
The 45th Edition of the Lyman Reloading Handbook also has a table showing Center Fire Rifle Velocity Vs. Barrel Length. Their figures apply to barrels between 20 and 26 inches in length and agree with the Remington figures. The Lyman table shows the following approximate velocity changes:
For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
For rifles with muzzle velocities in the 2001-2500 fps range, the change in velocity for each 1" change in barrel length is 10 fps.
For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps.
For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps.
The 43rd edition of the Lyman reloading Handbook gave some concrete examples of velocity loss for specific calibers and loads. The Lyman technicians chronographed some high velocity cartridges in rifles with barrels ranging in length from 26 inches down to 22 inches with the following results:
The average loss for the .243 Win./100 grain bullet was 29 fps per inch.
The average loss for the .264 Win. Mag./140 grain bullet was 32 fps per inch.
The average loss for the .300 H&H Mag./220 grain bullet was 25 fps per inch.
For standard high intensity cartridges in the same test, the Lyman technicians chronographed the cartridges in barrel lengths ranging in length from 24 inches down to 20 inches with the following results:
The average loss for the .270 Win./130 grain bullet was 37 fps per inch.
The average loss for the .270 Win./150 grain bullet was 32 fps per inch.
The average loss for the .300 Sav./180 grain bullet was 17 fps per inch.
The average loss for the .30-06/180 grain bullet was 15 fps per inch.
The average loss for the .35 Rem./200 grain bullet was 11 fps per inch.
After a bunch of disclaimers, the Lyman people concluded, "The rule of thumb is that high speed, high pressure cartridges shed more speed in short barrels than do the low speed, large bore types." It's funny, but that is what I had suspected all along!
