I realize what follows goes without saying but it bears reviewing. The best retention tactic with any firearm is environmental and situational awareness: muzzle proximity to an adversary or blind obstacle; forceful verbalization to an adversary who's confronted of the consequences of failing to comply; and maintaining the ability to disengage or withdraw.
Should awareness fail then the next best tactic is that advocated by Spectre, using the firearm for its' intended purpose. There are two caveats here. First, that you're going to have to be able to articulate to investigating authorities the reason for that confrontation and that a fear of being disarmed caused a fear for your life so great that pressing the trigger was the only thing you could do. Second, you and yours must be prepared to deal with the potential ramifications, personal and legal, that may follow.
If a struggle over a shoulder weapon does occur then the primary usefulness of a sling is to keep the firearm attached to its' rightful possessor. Either a 2 or 3 point sling can work here but the advantages to the 3 point are more rigorous retention and the ability to bring the weapon on target again quickly.
Physical retention tactics must be simple, practiced, and effected reflexively and immediately with power. If the release technique is unsuccessful then the goals of the physical tactics must be to keep the weapon "wrapped" into your body and to control the muzzle. In a "team" confrontation in which 1 member becomes engaged in a struggle over a weapon, countermeasures should include instantaneous announcement of the disarming attempt to the other member(s) to permit them to attempt an intervention.
Early in my career when the issue weapon was the revolver, a last resort that was taught to avoid having to shoot someone was the "muzzle strike" to the center of the face. This was designed for use against an adversary from whom you were able to disengage after their first attempt to disarm you but who contiuned to close to continue the attempt and it presumed by this time that the weapon was now in your hand. The principle consideration using a muzzle strike with a fixed barrel weapon is proper indexing of the trigger finger outside of the trigger guard. It's been my experience that a sling properly mounted to the body, and used for retention, precludes an effective muzzle strike.
Finally (gee, it's about time you're probably saying to yourself), if the sling is meant to maintain control of a shoulder weapon during a disarming attempt then I'm not sure that it makes much sense to "cut away" this primary weapon into the hands of the adversary to try to access a secondary weapon, even if you do have one.
Miratio, Incursio et Sestinato.
[This message has been edited by SKN (edited 12-02-98).]