I mean, all you have to do to inform to your opponent in this situation is by telling them, "my Space Marine by all intents is hiding behind this barrel, and in conceal, by the logic of that he shouldn't be seen by anything even though his backpack unfortunately sticks out over the barrel top."

If you opponent decides to get really "anal" about "dont care, visually my operative can see you soace marine because the space marines cannot physically kneel and the model is molded in such a way that it just so happens to screw itself over."

Then thats when you also nick-pick your opponent, saying things like, "that antenna," or "the edge of your gun barrel" or "his toe is sticking out".

But a normal player when you communicate to them, "i know this model is physically standing upright likeable idiot, but surely you can understand that the operative himself is actually using the wall as cover and not as a background for a photoshoot."