Because of a bum hip, I can't use any kind of sitting position. NRA rules state that if a shooter can't use the prescribed position, he can use an alternate, provided that the alternate position is "less stable". So I had to kneel.
BTW, standing IS NOT the same as offhand. Jeff Cooper has a short description of the differences in one of his books.
An attached sling in the standing position needs to be tight. The pendulum action of a loose sling causes an astounding amount of front sight wobble.
I shot the CMP courses for several years with just a T-shirt or a hoodie, or a light fall jacket. I wanted the matches to imitate the way I'd be shooting in the hunting fields.
Once I got into the sport, I decided if I was going to play the game I needed to Play The Game, and got a Turner Saddlery sling and a Creedmoor Hardback.
Scores went up 20 points in a 500-pt aggregate match.
Shooting glove soon followed.
Spent 4 years in Marksman class, then in 2 more (after the equipment upgrades), I made Sharpshooter, and almost made Expert.