I'm sure that's exactly what they tell you. Creature, I have no doubt you're a much better shot than I, and I am not here to flex my e-trigger finger. However, I think you fundamentally misunderstand what your Marine shooting teammates are telling you. I'd like you to do me a favor. Ask them if they are talking about the contact patch, where a bony part of the body merely contacts another bony part, or if they are talking about building a platform, a scaffold of bone, where the bones stack up from the ground to the weapon in order to create a stable platform and resist the effects of gravity and minimize muscle tension. They are, I'm sure.
I'm a ridiculously flexible man, always have been. That said, at first it was painful for all of us to crank down tight and hold our positions, as I'm sure it used to be for you. We were always encouraged to crank the sling up to the tightest possible degree, and then naturally align the body towards the target. In kneeling, that means sitting on my foot as it lay flat on the ground, not up on the ball of my foot, and with the forward foot tucked back against my crotch rather than placed out towards the target. If I were to place my elbow directly on top of my knee cap the position was much less stable than when I was tightly cranked into the tightest position possible. When I cranked down into the sitting position, for example, I tucked my legs up tight into my crotch, and rested my elbows into pockets created by the inside crook of the knee, never on the bone of the knee itself. Same thing with kneeling, the tricep, not the elbow, contacted my knee. I never concentrated on ensuring the contact patch was a bone on bone relation, in fact, I virtually always avoided it.
I don't know, it worked for me, I was 2nd highest in my series in boot and never shot less than 231 on the KD course, including pre-qual.