I see the most mechanical shift in ARs when shooting in the prone on a bipod. This comes directly from how much torque you impart on the bipod as a shooter.
Before I even explain that, I'll clarify that I do not hard load bipod legs when shooting groups or zeroing in the prone. I learned that lesson many years ago at a zero event at a military competition. You can't always guarantee in every position that you can drive the gun in the same way that you would in a hard bipod load, solid support bag. It's kind of an artificial position. If you're shooting off a barricade/ bag, tripod, or what have you it's not likely that you can trap the gun in that same "vise" type of hold. Hell, if you shoot off a polished concrete floor you can't load the bipod like you would if you were driving it forward into a half sunken 4x4. So I zero with a neutral load because it's more reproducible from position to position. If anything, I'm not driving my shoulder into the buttstock but pulling back on the pistol grip to pull the buttstock into my shoulder. To be honest, I shoot the best 100yd prone groups with a very light, neutral hold which is still a little bit artificial because you can't manage recoil that way in alternative positions. But all of that is positional shift. Back to mechanical shift.
Even with a neutral bipod load, light shoulder pressure and light or firm shooting hand grip, you can impart flex on the fore end by bearing down with cheek pressure. If the toe of the buttstock is anchored pretty good in a firm bag like a heavy fill game changer and you place a lot of weight or pressure from your head with your cheek(just forward of the rear anchor), that will impart torque on the bipod and cause flex in some fore ends. Especially 16" long handguards with the bipod as far out as possible. I've found that combined with varying amount of shoulder pressure or contact it's a recipe for wierd high and low shots or vertical stringing. In my experience shooting from a bag on a barricade or on a tripod both have the same mechanical influence on the handguard. The forward anchor point is too far back on the handguard to flex it. If there's a shift there it's more than likely positional. If it's vertical shift it's an angle of departure effect caused by differing amounts of recoil management. If it's horizontal or diagonal it's likely head position.
The latter is why I like to use a taller HOB. Having your head in a more vertical, upright posture in the prone is going to transition a more similar eye relief and head position to kneeling or standing. If you're super low in the prone position your neck is elongated in a horizontal attitude and the base of your skull is contorted at a more right angle to your neck line. As you elevate in shooting position your neck and head align more naturally vertical and this undoubtedly has an effect on eye relief and head position. I think if you're shooting with eye pro you're doubling down on this effect by changing the ocular position your eyeball is in. From extreme angle just under your eyebrow and top of your undoubtedly angled eye pro lenses to a more centered in the eye socket, directly straight out, and perpendicular to the flat plane of the lens.
To complete not-answering your question
I just bought a M4E1 upper and handguard and installed it on my 22 ARC. So far it seems to be reigning in some oddball flyers. I only have two range sessions with it so haven't really proven anything yet.
, while proper fundamentals and well fitted rifles solves the other.
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