Question for one of you mathematicians.

[Boz330]

I need the formula to figure sight adjustment for fixed sights. The underhammer that I built hits about 15" high at 75yds. I have the highest front sight I could find but need to mill the receiver down for the fixed rear peep to get it on.


Thanks
Bob

[JeffinNZ]

https://floydpics.wordpress.com/2016...t-corrections/

[map55b]

Sounds like you need to lower the front or raise the rear.

[dondiego]

Lowering the front will make it shoot higher.

[aspangler]

Brownell's has a sight calulator on their website.

[Ballistics in Scotland]

Sounds like you need to lower the front or raise the rear.

Lower the muzzle, yes, by raising the front sight. The OP has mentioned fitting the highest, but some kind of ramp silver soldered to the barrel would do it.

Taking the round figure of a yard from front sight to aperture, one sight ought to be a sixth of an inch different. Rather than milling the receiver down (which you would regret if you find another type of rear sight you like better), it looks like you could mill the bottom of the sight up, and screw it down lower. Or if the sight body is steel, it could be bent down, hot if it is hardened steel) to closely fit the curvature of the receiver.

It would be a good idea to finish the development of your working load before you finalise the sighting. Bullet weight, even more than powder charge, can make quite a difference to the point of impact. It isn't simply in the trajectory, but in the barrel flip and at what point in that flip the bullet leaves the muzzle.

[map55b]

Maybe a picture will help/

Leave the front sight where it is(blue line and sights), you would need to raise the rear to the purple height to get it in line with the bore or you could leave the rear sight and lower the front to the red height to get it in line with the blue front. Of course you could meet someplace in the middle.

This might help too: https://www.brownells.com/.aspx/lid=...ght_Calculator

[Boz330]

Lower the muzzle, yes, by raising the front sight. The OP has mentioned fitting the highest, but some kind of ramp silver soldered to the barrel would do it.

Taking the round figure of a yard from front sight to aperture, one sight ought to be a sixth of an inch different. Rather than milling the receiver down (which you would regret if you find another type of rear sight you like better), it looks like you could mill the bottom of the sight up, and screw it down lower. Or if the sight body is steel, it could be bent down, hot if it is hardened steel) to closely fit the curvature of the receiver.

It would be a good idea to finish the development of your working load before you finalise the sighting. Bullet weight, even more than powder charge, can make quite a difference to the point of impact. It isn't simply in the trajectory, but in the barrel flip and at what point in that flip the bullet leaves the muzzle.

The front sight is dovetailed in so the ramp would need to be dovetailed as well which is an option although a bunch of work since I would need to make it from scratch. I really didn't think of that though.
I thought of milling the bottom of the sight but I'm not sure that I can get enough to bring it in. That is why I wanted the formula to see how much it would take. I did bend the rear sight a little but it has already started to look oval from the angle. It did drop the group by about2 inches. Load development isn't an issue since the group was less than 2" for 3 shots and that will work for me in a hunting rifle. The gun has a round ball twist but in a similar rifle I tried to shoot a REAL boolit without much success. Within my self imposed range limits the (50cal) round ball should work fine for deer.

map55b, lowering the front sight would raise the point of impact even more than it already is.

Bob

[Omnivore]

You don't need any “math" per se, and you don't need a special "calculator" or any such nonsense. You only need to do some multiplying and dividing.

First, you need to know your rifle's sight radius, so start by measuring the distance between the front and rear sights.

Now you simply determine the number of sight radii in the distance of 75 yards.

Example assuming your sight radius is 30 inches;

First you convert 75 yards into inches;

75 yards x three feet per yard, x twelve inches per foot = 2,700 inches

2,700 inches divided by your sight radius of 30" means 75 yards = 90 sight radii.

Now you divide the 15" Point Of Impact (POI) error by the number of sight radii in the distance to the target;
15/90 = 0.167

Since you've done all your figuring in inches, the answer is of course in inches (that's why we had to convert yards to inches), so you need to move one, or the other, or both sights, vertically, by a total of 0.167". It's up to you, so you could raise the front sight .167", or lower the rear by .167" or raise the front AND lower the rear, each by half that amount (about 84 thousandths up in front, and about 84 thousandths down in the rear) and get pretty much the same result.

It’s a simple matter of realizing that if you move your sight any amount, that amount will be multiplied by the number of sight radii between the gun and the target. Once you understand that, all future sight adjustments, or replacements, etc., you will be able to accomplish spontaneously.

That won't get you exactly spot on, but it'll get you very close; you have to shoot some groups and then do your fine adjustments in the field with a file, and shoot some more, etc., to get it spot on at your desired distance. Being practical then, I'd add, say, two tenths or so, to the front sight and then go out, taking a file with me, and shoot.

Of course you’ll want some room for adjustment, so don't get it all zeroed with no adjustment left in that rear sight. If both of your sights are non-adjustable, make sure your front sight is taller than called for in the numbers, so you can file it down a tiny bit at a time, during actual shooting trials, until you get it just right at the desired distance while using the desired load.

AND OF COURSE you wouldn’t start into any of this process until you’ve decided on a load that shoots the best groups. To get the sights all dialed in, and then change to a different load that shoots to a different POI, is to waste your efforts.

[Omnivore]

To pretend that we're doing "math" we could put all into code, so it looks like bloody gibberish;

Delta POI = D/SR x Delta SP
where POI = Point Of Impact, D= Distance to the target, SR = sight radius and SP = Sight Position. Delta of course means "change" and is oft represented mathmatically as a triangle. Dumb stuff like that. Then we can smugly relate it all in such code, and then smugly walk off, feeling superior, when no one understands it. THAT is "math". We don't need no stinking "math".

[Omnivore]

Not that I'm recommending it, mind you, but it almost looks like you could bend that rear sight down a bit.

[Boz330]

Omnivor, thank you that's what I need. I took a lot of math that I never used, 52 years ago. I did bend the sight but it didn't get me much and angled the sight up in the air enough that the peep looks like an oval rather than a round hole for sighting.
I've filed many a front sight to get it on so I'm familiar with that process. The original sight on the rifle was a notched rear, but my eyes don't deal with them quite as nice as a peep, which was after market and not matched to the front.

Bob

[Omnivore]

OK. Yes, I agree that a peep, or aperture, sight is superior to a notched blade. It should present a round aperture to the shooter though-- Any bending would have be in the form of creating an "S" curve in it, such that the aperture is aligned with the front sight. Otherwise I'd raise the front sight a bit more.

Let us know how it works out.

[rhbrink]

That is one of the actions that has three set screws that hold the barrel I wonder if the receiver hole is drilled out of alignment? Maybe the breech plug is not straight or tapered on the wrong side causing to barrel to point up instead of straight? Would it be possible that you could remove the barrel and check to see if you could add a shim or maybe even add metal to the stub of the breech plug and then file to fit to move the barrel in the right direction? I had one that was drill out of alignment to the right and was able to fix it took a gunsmith buddy with a mill to ream out the receiver hole and sleeve it to bring everything back into alignment the front of the action was not square either and I'm sure that this can come into play also.

Good Luck

[William Yanda]

If your self imposed limit is 50 yards, why are you sighting at 75? Range set up? Not trying to hijack, just curious.
Bill

[Boz330]

If your self imposed limit is 50 yards, why are you sighting at 75? Range set up? Not trying to hijack, just curious.
Bill

Self imposed limits is 75yd rifle is (50cal). I forgot to put a comma behind limit.

Bob

[ogre]

I like the looks of that peep sight Bob. Where did you come by it?

[Tatume]

This is an example ratio and proportion, x/y = a/b . If you know three of these things, you can easily solve for the fourth. Set it up so your unknown is x, then x = ay/b. That is x/30 = 15/2700, or x = 15(30)/2700 (75 yards converted to inches is 2700 inches). x = 0.167 inches.

The principle is useful in many situations, which is why I expounded on it. Ratio and proportion can be used for solving so many problems, that it may be the single most useful mathematical tool there is.

[Boz330]

I like the looks of that peep sight Bob. Where did you come by it?

Muzzle loaders builders supply. They come in steel, brass, and stainless.

Bob

[pietro]

The front sight is dovetailed in so the ramp would need to be dovetailed as well which is an option although a bunch of work since I would need to make it from scratch. I really didn't think of that though.

The Williams Gun Sight Co makes/sells their "Shorty" 1/4" wide front sight ramp in several different heights, which is available with an adaptor that locks the ramp to a barrel dovetail with a screw down through the floor of the ramp's dovetail slot, and which also takes pretty much any height front sight blade.




Alternately, you can raise the front sight you have with a "riser block" (google), which is a block the has a male dovetail on it's bottom to fit your barrel's sight dovetail, and a female dovetail on the top fit your existing front sight blade.






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