OK folks, since I've just registered and this is my second post, let me throw out a few details that I base my comments on:
-I've shot smallbore rifle (3 position - Olympic style) for thirty years
-I currently coach at the college level
-I've shot highpower off and on
-I recognize that the above are all "games"
-I recognize that many fundamentals are best learned/taught through these games
-I've taken Practical Rifle (and pistol) under Jeff Cooper at Whittington
-I've also trained with others (eg. Louis Awerbuck)
-I've read most of Jeff Cooper's writings
-I know how to use a sling (Olympic, 1907, Ching, hasty)
-I have an open mind and look at things objectively
-I have corresponded and dined with Cooper, but I don't "worship" him
-I have and love a Steyr Scout, but also play with pseudo scouts, M1s, etc.
-I tote a rifle in the woods whenever I can, which ain't often enough....
All of that said, I think the essence is that IF (big "if" here) you are doing things correctly, then a sling will not help in offhand/standing.
My reasons for saying this:
1. If you can get lower, get lower
2. If you can get closer, get closer (OK, both from Cooper)
3. Bone supported sling positions should be used whenever possible
4. If you MUST stand, then it's for one of these reasons:
a. It is a snap shot - no sling will help this
b. It is a moving shot - no sling will help this
c. It is an unrushed shot and you cannot find or get to a support
In c. above, a proper (again, a big leap) standing position will yield a better shot than trying to use a sling. Use of a sling as many have described will result in an increase in muscle tension - BAD for shooting stability. You must be as relaxed as possible for maximum accuracy.
Having said all of this, some have stated specific cases where a sling may actually stabilize. One such case being during rapid fire, which from standing by definition will not be accurate and has no real world use that I can think of (even in the military - covering fire of that type does not have precision as it's main goal). While it _may_ help if you are shooting rapid fire at drink cans at the bottom of a berm (and maybe not), while fun, this accomplishes nothing. I will submit that it won't help you train for either a real snap shot or a deliberate standing shot, both of which have real world applications. I'm sure that it's great fun, though. Nothing wrong with fun, it's just not important to Cooper, to whom rifle work is very, very serious.
As a personal comment, I have noticed a "dampening" effect when standing with a sling used around my back and left arm (bastardized combo of team sling and hasty sling). I do intend to test this some time to see if it really helps. I expect that for deliberate standing fire, it will not help, but for a standing "pray and spray" approach, it will help get back on target faster and perhaps tighten things up. I just can't see where that latter buys you anything from a practical perspective. I will, however, evaluate it objectively and report if I learn anything new from it.
Keep the perspective of the statement in mind here. As many of you know, often when Cooper speaks/writes, he does not fill in all of the details. If it doesn't matter to what he's commenting on, then he leaves it out, expecting the objective, thinking reader to be able to discern what is germane and what isn't. He tends not to comment much on what is not germane. Thus you may see no comments whatsoever on areas that he can't see as productive. I think that some of this is going on with the current subject.
All of this is of course MHO, but with the notation of some experience as stated above. I hope that no one takes anything as criticism, merely my view of the issue. Pardon the length of my contribution. This is a subject that I care about a great deal, and will continue to study.
Also, I really love shooting standing!