Effects on POI from pressure on non-FF handguard

[Arctic1]

As to not derail the "Grouping" thread further, I'll post this here:


No. The flexing is contained between the weight of the shooter and the rest. There may be some flex at the upper, but it is minimal in relationship to the rear sight. Most of the flexing of the barrel will be between the chamber and the rest. Ahead of the rest will remain straight. Even so, the bend would be a parabola, not a straight curve and the barrel will be straight at the muzzle. Also, the bullet will depart at an angle inline with the bore at the muzzle.

The shooter aligns his sights with the target, then leans their weight on the rifle. This bends the barrel, deflecting the barrel upward. Left in this position, the bullet will fly high. However, when looking through the rear sight, the shooter sees the front sight is high and angles the muzzle downward to bring the sights back on target. This angles the bore downward, causing the bullet to fly low. It has the same affect as either a) raising the front sight, or b) lowering the rear sight to drop the point of impact.

Take a piece of PVC pipe and fix it with something to act as sights at either end and line it up without flexing the tube and look at the angle at the "muzzle". Then without changing your position, bend the pipe with the curve pointing towards the floor and again align the sights with your eye on the same target. You will see with it bent so, the "muzzle" end of your pipe is angled downward in compared to when it was straight

I really cannot see how exerting pressure on the handguard while resting it on sandbags etc., thereby pushing the barrel upwards, will cause a downward shift of POI.

The change in LOS is minimal, and there will be no need to tilt the gun downward to align the sights. Most will adjust their head, not the gun. And if the barrel is moved due to pressure on the handguard, the bore axis must be altered. There is no way around it.

I have run this through my head several times, and I cannot see how it is physically possible. Also, all accounts I could find listed shooters having issues with a higher POI shooting off a rest, compared to shooting offhand. Ie, they zeroed from the rest, and when they shot off hand, the POI was lower than the zero at the same range.

[MistWolf]

...The change in LOS is minimal, and there will be no need to tilt the gun downward to align the sights. Most will adjust their head, not the gun...

Change to LOS is minimal just as the change to LOS is minimal when raising and lowering the front sight normally. But that small change in angle adds up over distance.

With pressure on the forearm deflecting the muzzle upward, it raises the front sight, which means it's no longer centered in the aperture. If the shooter simply re-adjusts his head to re-center the front sight, the front sight will be above the target. In order to bring the sights back into alignment with the target, the shooter must move the rifle, in this case, downward. If, during all this monkey motion, the shooter changes the pressure, it will again change point of aim and point of impact


And if the barrel is moved due to pressure on the handguard, the bore axis must be altered. There is no way around it...

Of course the bore axis is altered. That's why the POI changes in relationship to the POA

[Arctic1]

Change to LOS is minimal just as the change to LOS is minimal when raising and lowering the front sight normally. But that small change in angle adds up over distance.

True, but here both the LOS and bore axis is moved. When mechanically adjusting the front sight, ie adjusting the front sight post up, the bore axis does not change along with it. The only change is in the LOS, resulting in a need to adjust the rifle.

I don't see why an upward movement in both LOS and bore axis would cause a downward shift in POI. It does not make sense.

[MistWolf]

Let's put this on pause while I work up some illustrations

[MistWolf]

First, it's a good thing to keep an open mind. It doesn't do to try to bend the laws of physics to suit one's views of the world. In that path lies eternal denial.

We'll start by creating a little experiment to prove my assertion. The blue PEX tubing represents the bore and line of departure for the bullet. The line in the cement is line of sight. The two vertical PEX tubes represents the front & rear sights and the roll of gaffer's tape is the eye of the shooter. We're using a little black tool bag to represent the sandbag
http://i115.photobucket.com/albums/n289/SgtSongDog/Research/DSC_0003_zpsd06faaa5.jpg

Pretty cool, eh?

Now, let's put a bend in the blue PEX to show in an exaggerated fashion just what happens when the weight of the shooter is placed on the rifle. Just as expected, the muzzle is deflected upward. We then tilt the blue PEX to realign the sights back on target and
http://i115.photobucket.com/albums/n289/SgtSongDog/Research/DSC_0004_zps32a58be5.jpg

much to my surprise, the line of departure is above the line of sight. The shot would indeed strike high, proving my assertion wrong. There you have it.

Sometimes a guy can work a theory out in his head and sometimes a crucial detail gets over looked. I should have realized I was missing something when the image of that German carbine with the barrel bent to shoot around corners kept popping into my head. I blame it on dyspepsia- I mean dyslexia.

Arctic1, the day is yours. Congratulations and well met!

[Agnostic]

Thanks for your efforts to illustrate the scenario, I learned something. And thanks to both of you for showing us how to disagree with respect and an open mind.

[Arctic1]

Thanks for taking the time to set up the "gun" to illustrate the mechanics of it MistWolf.

As for the discussion, it was really about making sure I hadn't misunderstood something as much as it was about getting the correct info out. The most important thing is to learn.

[MistWolf]

Once I saw where I made my mistake, I felt pretty embarrassed. If you hadn't insisted on a clear explanation, I would have ended up an even bigger fool before figuring it out