When I started shooting sling-n-irons, everybody preached using the right-knee-up position. David Tubb used it, Nancy Tompkins used it... everybody who was winning at the national level was using that position (trivia: I believe the Romanians actually debuted that little variation many, many decades ago and cleaned everyone's clock at a few international level smallbore prone events, and it took hold from there).
Problem for me was, that particular position was pure agony for a 20 shot string. And I don't mean in a "with practice, your muscles will get used to it" sort of way. I ended up using a position with my legs straight behind me, and if anything, my left leg up just a little. Unconventional, for sure, and I wasn't in danger of taking any national titles... but it was repeatable and comfortable enough I could stay in it indefinitely.
Not too much later, I came across a book titled 'Ways of the Rifle', put together by a host of top-level smallbore and air rifle coaches. The interesting bit is that in the prologue, they discussed how they'd originally intended the title to be 'The Way of The Rifle', as in this is *the* way to shoot Offhand, this is *the* way to shoot Kneeling, this is *the* way to shoot Prone. But when they sat down with all their collected photos of world and Olympic champions, the positions were all over the board. Far more than just a few exceptions to the 'rule' for each position.
In the end, they deduced that there *were* some guidelines that could be reasonably established... but they had more to do with body types. Women (in general) were more flexible, but less able to hold position by dint of sheer mass. Even among men, people with tall, thin builds had different basic positions than others with short, stocky builds. In short, one-size-fits-all cookie-cutter positions were maybe a good starting point, but were rarely the final destination for the top shooters.
Even when my own shooting transitioned to primarily F-class (and specifically, F/TR), we found that somethings worked better than others. The right knee up position was popular initially, especially with the people crossing over from long-range prone competitive sports. You can shoot very accurately that way. But... but... here's the thing. Unless you use a significant amount of pre-load, the muzzle will tend to want to move *away* from the right side, where you have effectively 'reinforced' your position with that right leg up, and causes your muzzle to 'hop' to the left. Almost every time.
Moving to legs straight back will reduce this considerably, and with the use of muzzle brake (not allowed in F-class) may be all that is required to get the gun to track consistently back on to the target. Without a brake, I've found that pivoting the lower body a bit more to the *left*, almost to where the right leg is straight in line behind the bore, or *very* slightly left, facilitates tracking right back on target.
The catch is... as mentioned at least a couple times previously... this is all done on (mostly) flat/square KD ranges, and becomes simply 'good theory' once you get out in the field. Muzzle brakes, pre-loading, and gripping the gun more firmly are all valid tools once you leave the range behind.
