Not a DI, but I've had a lot of experience with them, and shooting.
When I was in the USMC the sling is NOT used in the standing position during qualifications.
The following is from the USMC Rifle Marksmanship Manual MCRP 3-1A which seems to include the sling. The manual is dated 23 Feb 1999, so maybe they allow it now.
5008. Standing Position
a. Description. The standing position is the quickest position to assume and the easiest to maneuver from. It allows greater mobility than other positions. The standing position is often used for immediate combat engagement. The standing position is supported by the shooter's legs and feet and provides a small area of contact with the ground. In addition, the body's center of
gravity is high above the ground. Therefore, maintaining balance is critical in this position.
b. Standing Position with the Hasty Sling. Apply the seven factors to this position (paragraph 5003c). To assume the standing position with the hasty sling (see figure 5-40):
Figure 5-40. Standing Position with Hasty Sling.
* Square your body to the target.
* Spread your feet apart to a comfortable distance with the left foot slightly in front of the right foot. This distance may be wider than shoulder width.
* Distribute your weight evenly over both feet and hips. Your legs should be slightly bent for balance. Balance will shift forward slightly to decrease recovery time and increase the stability of the hold.
* Place your left hand under the handguard.
* Grasp the pistol grip with your right hand and pull back to place the rifle butt in your right shoulder pocket. Apply rearward pressure with your right hand to hold the rifle butt in your shoulder.
* Hold your right elbow in a natural position.
* Bring the rifle sights up to eye level instead of lowering your head to the sights and place your cheek firmly against the stock. Ensure your head is erect so the aiming eye can look through the rear sight aperture.
* Move your left hand to a location under the handguard which allows the sling to support the weapon and the front sight to be centered in the rear sight aperture. To adjust for elevation:
* Move the left hand rearward or forward on the handguards (moving the hand rearward elevates the muzzle).
* Open or close the “V” of the left hand for small adjustments (closing the “V” elevates the muzzle).
* To adjust for a cant in the rifle, rotate the handguard left or right in the “V” formed by the thumb and forefinger.
* Adjust your position to adjust sling tension. Moving your left elbow out tightens the sling.