Sight geometry question

[Brokennock]

I need some help from the math and engineering guys.
All else being equal,hypothetically, if one's gun is sighted in at, say 50 yards, and one moves one's rear sight 6" back toward the breach plug, does this change the point of impact?
If so, up or down?

 

[painter]

I don't know if it will make you shoot higher, or lower, but here's the formula to correct...

Amount of Error X Sight Radius​	= Sight Correction Needed​
Distance to Target​

 

[Loyalist Dave]

I need some help from the math and engineering guys.
All else being equal,hypothetically, if one's gun is sighted in at, say 50 yards, and one moves one's rear sight 6" back toward the breach plug, does this change the point of impact?
If so, up or down?

All other things being equal, it should not move the point of impact at all.
I write "all other things being equal" for in the case of a swamped barrel, you may be elevating or lowering the rear sight in relation to the front sight...., but this may be quite slight, and depends on the amount of the taper on the swamped barrel. In the case of a tapered barrel (sometimes found on a hand built replica plains rifle or an English sporting rifle ) by moving the sight toward the rear or moving it toward the muzzle, you are also elevating or lowering the height of the sight in relation to the front the sight.
Colerain Tapered Barrel Illustration

You may find that you get a bit more of an obvious elevation or depression of the point of impact by varying the powder charge or granulation and thus changing the muzzle velocity. This change due to powder might also be used to "correct" a point of aim from the movement of the sight....so if you moved the sight backwards but then dropped the powder from 90 grains to 80, you might have mitigated any change....

The moving of the rear sights and/or a change in the powder load will be amplified the farther away from you the target sits. Thus a ½" elevation at 50 yards might be a full inch higher at 80-100 yards, OR...., might actually "correct" by the time the ball gets to the 100 yard mark as the ball has slowed in flight and "dropped" a bit. So a lot depends on the MV as well as sight placement.

The other effect you might encounter is a tightening of the groups when moving the sight backwards, regardless of a noticeable change in elevation. The farther the distance between the sights, aka the longer the sight plane, tends to make it easier for the human eye to more consistently align the sights. Go to a peep sight and this may be further amplified.

On the other hand I've seen older shooters move the rear sight forward due to focus problems, and although the sight plane was shortened, the fact they could then see the sights well, made a large improvement in their accuracy. Not an actual improvement in the rifle, but in what their eyes could do for them.

LD

 

[Larry (Omaha)]

I need some help from the math and engineering guys.
All else being equal,hypothetically, if one's gun is sighted in at, say 50 yards, and one moves one's rear sight 6" back toward the breach plug, does this change the point of impact?
If so, up or down?

A pair of sights is nothing more than a reference in conjunction to POI. Basically, if you have a parallel barrel, there should be no sight compensation needed. In respect to moving 6" to the rear and dealing with taper or swamp the change would raise the sight (reference) and POI.

 

[Bob McBride]

As stated above, on a straight barrel, no, assuming your eye position stays the same. A swamped barrel will raise the rear sight causing the line of sight to be dropped in relation to the barrel causing your POI to be higher after you compensate by raising the muzzle.

 

[Pete G]

If you raise the rear sight it will raise yor POI, requiring the front to raised an equal amount.

 

[Brokennock]

Thank you.

L.D. actually the sight is very blurry, but I am a very "focus on the front sight" shooter, even when a rear sight is positioned where I could see it clearly, I don't. Just in shouldering the gun the sight is blurry but seems to focus me more on the top of the front sight, and, this being a smoothbore, I can completely ignore the rear sight if I want or need to.

 

[Scota@4570]

Yes, It will hit a little lower.

Assume a non swamped barrel. Normally the rear sight on a rifle is higher then the front relative to the bore. If you move the rear sight back you have a longer sight radius. The angle the bore is tilted up will become less.

To demonstrate this to yourself, draw two long thin triangles on their sides. The short side of the triangle is on your right. Use the same length right side for both triangles. Make one triangle twice as long as the other. The bottom of the triangle represents the bore. The top is the sights. The longer triangle represents moving the rear sight to the rear. The angle on the left side, the pointy end, of the triangles are different. The longer triangle has less angle on the pointy end, to the left. The bore is pointing up less, the bullet hits lower.

I did the math...the point of impact change will be 0.63" at 50 yards. That assumes a 30" sight radius and a 36" sight radius.

So, about five file strokes on the front sight, not much.