Perhaps the oldest and most respected sling design still in widespread use is the leather military two piece M1907 gun sling. While almost 100 years old this relatively simple design is still the benchmark by which all others are measured. It was originally intended to allow an infantryman to utilize the then new U.S. Rifle Caliber .30 M1903 Springfield to its fullest potential. It did this by bridging the gap between the simple loop competition slings used by target shooters and the common carrying strap. The result was a sling that could comfortably carry an infantryman's rifle over hill and dale. Then, once he got to where he was going, it could substantially increase his practical accuracy.
The M1907 worked so well that it soon became highly popular with hunters and target shooters as well. The question then becomes, how on earth can a couple strips of leather make me shoot better? While it at first seems improbable, the M1907 can significantly aid your shooting. It accomplishes this by tying the long bones of the arm together into a rigid brace. This provides a steadier hold, and thus increased accuracy.
Why use a loop sling for support? Simply put, a properly employed loop sling will dramatically increase your stability, and thus accuracy. Competition shooters regularly use nothing but a leather sling for support when shooting at 600 and 1,000 yards. How much does it help? Re-zeroing my AR-15 match rifle for this year's NRA High Power season I fired two 5-shot groups prone at 200 yards using a sling, shooting jacket and iron sights. The first measured 1 inch, the second 1.25 inches. So properly utilized, a sling can help a lot!
While it seems hard to believe that a strap of leather or cotton web can add support, it does indeed. It accomplishes this by forming your non-dominant arm and the rifle into a inverted triangle. The sling attaches above your bicep and runs to the upper sling swivel. This allows your arm to form a 'V', with your elbow resting on the ground or other support, but limits your forearm's forward travel. The sling effectively limits how wide the 'V' can open, depending upon its length. The rifle becomes the top portion of the triangle.
The key is to have the sling too short for the rifle to fit easily. In the case of our triangle illustration, the 'V' must be forced open and the top piece wedged in, making the fit extremely tight and removing any movement. The end result is a rifle tightly wedged into your shoulder with no movement. How tight? One unaccustomed to using a shooting sling will find it uncomfortable at first. This is normal. With regular practice you'll soon come to appreciate the feel of a tightly slung rifle.
The M1907 sling design looks simple but a great deal of thought by an experienced rifleman obviously went into it. The sling itself consists of two unequal length pieces of thick 1.25-inch wide leather, and two leather 'keepers'. The long piece (Long End) has a two pronged metal hook (Top Hook) attached at one end and two rows of adjustment holes run its length. The short piece (Short End) has a hook (Short Hook) on one end, adjustment holes, and a metal loop on the other. The metal loop joins the long and short pieces together and the sling's length is adjusted by snapping the hooks into the various holes in the sling.