I believe that among the most misunderstood aspects of choosing a firearm is choosing the barrel length best suited for your use. A novice wonders why there are so many configurations. The only real consideration is which is the best for your intended chore, and this is determined by the chore you will perform.
The barrel determines the length of the firearm more so than any other trait. The action is a certain length, and so is the grip or stock. A double-barrel shotgun has a compact action and may have a shorter overall length than the pump action with the same length barrel.
A .308 Winchester rifle with a short action will be shorter than a .30-06 rifle if both have 24-inch barrels. Some folks just like longer barrels. Others like a short, fast-handling rifle.
The longer tube will result in a more complete powder burn. This means greater velocity. Sometimes, even most of the time, a longer barrel exhibits less muzzle flash due to this full powder burn. As a result, the projectile has more velocity and should shoot flatter and deliver more energy with the same loading.
Another consideration is sight radius. The longer the distance between the sights, the more likely we are to get a good hit without as much chance of misalignment. This is the reason military rifles usually had long barrels until recently.
With optics taking the place of iron sights, a shorter barrel is acceptable. As armies and individuals made more use of mechanized vehicles, the move toward shorter firearms was inevitable. Even shotguns do not have to be long-barrel jobs to get the job done. We cannot accelerate a load from a short barrel, but we can control shot spread with modern choke tubes. A 21-inch-barrel shotgun is pretty handy for most chores.
While the Mosin Nagant and the Krag still had musket barrels, the new Winchester 94 featured a 20-inch barrel and retained plenty of velocity for most chores. At 100 to 125 yards in dense wood, the 20-inch barrel was plenty.
The .308 Winchester is popular today largely based on its efficiency even in modestly short rifle barrels. The .223/5.56mm rifle is at its best a long barrel that retains much of its 3,000 feet per second plus velocity. The cartridge is a great house-clearing round and effective in short-range combat.
The problem is that over 100 yards and certain 125 yards, the cartridge loses much of its effectiveness. This is especially true when the barrel is short, as in the present M4. Be certain to know the limitations of your cartridge. Modern munitions such as the Hornady Black and 77-grain loads that open up at more modest velocity offer real efficiency even at low velocity.
When it comes to handguns, the short barrel is generally considered a personal defense firearm and the long barrel a hunting firearm with lengths from 2 to 8 inches and more commonly 2 to 6 inches. The pocket guns are the worst performers, as the short barrel doesn’t allow for a complete powder burn, making the velocity less. The short radius is more difficult to use as far as accuracy goes. However, these guns fit in a pocket.
An interesting compromise is to go just a little longer. I recently tested the Winchester 125-grain Silvertip in a Chief’s Special with a 2-inch barrel. Velocity was 840 fps. Moving to a Detective Special with 3-inch barrel, velocity was 890 fps, a considerable advantage. The point is, if the piece is a holster gun, the extra inch of barrel doesn’t matter and the handgun will exhibit greater velocity.
When it comes to concealed carry I think that most folks believe the 4-inch barrel is the outer limits. There is some validity to this opinion. I believe there isn’t a better-balanced revolver anywhere than the 3-inch-barrel Smith & Wesson Custom Shop .357 Magnum—unless you handle a 5-inch-barrel Military & Police .38 Special. This is among the finest revolvers I have ever owned as far as heft and balance are concerned, and it is fast on the draw from a shoulder holster.
Still, a 4-inch-barrel revolver is the upper limit, and shorter revolvers are often faster handling. This all came about when we moved to vehicles from horses. We needed a shorter, faster-handling handgun, and this is still true today. A soldier deploying from his troop carrier needs a short rifle, and a concealed carry permit holster fighting off a carjacker needs something that handles quickly. Ballistics do not matter as much.
In self-loading handguns, the equation changes. The barrel may extend from the slide. The Springfield XDM 4.5 with a 5.3-inch suppressor-ready barrel exhibits excellent velocity. I have carried a 5-inch-barrel Government Model 1911 for many years. It isn’t much of an adjustment to carry a Glock 34. As my friend John Nuckolls pointed out, one is about as long as the other, and the Glock is much lighter.
It isn’t necessary to give up that much in barrel length with the self-loader if you have proper leather gear. Choosing the proper barrel length is a consideration of weight, energy, sight radius, and the ability to comfortably pack the firearm.