Sights & Point of Impact Geometry

[eagle24]

I'm taking a new rifle to the range tomorrow.  It has been pointed out to me that the rear sight is probably high and I suspect it is going to shoot high.  Is there a formula for calculating roughly how much to take off the rear sight to move the point of impact at a specified distance?  Seems like I have seen a formula somewhere, but I haven't been able to find it.  I could always do the trial and error and take it slow, but I just wanted to have an idea before I started filing.

[Dphariss]

I think the formula is.
amount the impact needs to be changed times sight radius divided by the distance to target in inches.

So if you want to change the impact point 1" at 100 yards with a 36" sight radius you change the sight .01.

1*36/3600=.01"
2*28/1800=.031

etc etc.

Dan

[Daryl]

 Taylor and I've been changing sights and building new guns with new sights for decades, now, and have never reverted to math. of figuring "how much".  Trial and error works, every time.  We also do an educated guess at a zero - with front and rear sights level so if needs be, a 'bit' needs to be taken off the front sight.  Having to adjust the height of the rear sight's notch or top surface is painfull compaired to merely filing a tich off a thin blade.
The smaller the calibre, the more chance even sights will give a perfect 25 yard zero and be about 3/8" high at 50 yards.  Larger bores need a bit of difference between front and rear- to about 1/16" for the same sighting- all dependent on the powder charges used and the height of the sights above the bore.

Too- find an accurate load before filing sights. Sometimes guns shoot higher with more powder and/or tighter loads, sometimes they shoot lower with the addition of more powder.

[northmn]

Like Daryl, I never found anyway to find a measurement to a correct sight height.  Trial and error is slow but works very well.  Usually for me the front sight seems lower than it should be.  I usually get them close, preferably a little high and then shoot a bit before fine tuning.  I liked to take my time and do so on more than one day as I think fatigue can enter in.  After doing so I am willing to bet Daryl and Taylor get a feel for how much to remove also.

DP

[Long John]

Since the tragectory of a rifle ball is NOT a perfect arc or parabola section the ratio of change in elevation over distance to target, to change in sight height versus distance between sights is only approximate.  But it will get you into the right ball park at moderate distances with reasonable charges. 

Take the change in elevation desired and divide it by the distance from the rifle to the target.   Then multiply that number by the distance between the sights.  The result is the change in sight height you  need.  Keep in mind that all measurements must be in the same units of measure!  The distance to the target must be expressed in inches if the distance between the sights, change in elevation and change in sight height are expressed in inches.

Best Regards,

JMC

[Daryl]

With normal 36" to 42" barrels, we usually file an angle on the rear of the blade, then take the top down to that the bottom of that fresh angle. The angle filed on the rear top of the blade is usually about 1/64" to 1/32", sometimes even 1/16" deep or high, depending on barrel length and amount of adjustment needed. The shorter the barrel, the less is removed for the same overall group movement on the paper.

Suggestion - fire at least 5 shots, preferably 10 before changing the sights.  Also, use a target which gives you the best accuracy from the rifle, be it a solid square, solid round or hollow square or round, ringed or whatever. Different targets visually, will give you different point of impacts, depending on how you 'see' the target under the sights.

Going too far merely means having to file some off the rear sight, or learning to bury the front sight in the rear sight's notch for close ranges.

[eagle24]

I agree with you Daryl and if I were more experienced as a flintlock shooter and iron sights I wouldn't have asked.  Mainly, I just wanted to get an idea of how much was going to have to come off before I started filing.  For instance, if the rifle were to shoot 2" high at 25 yards are we talking about taking the rear sight down 10 or 12 strokes of a file? or 1/16"?  That's kind of what I was wanting to know.  I definitely think the rear sight is high and will have to come down.  I knew it when I got it made and on the rifle, but figured there was no reason to file it down until I shot it.  Who knows?  The barrel may have some runout or something and it may not hit as high as I think it is going to.  I will definitely find a hunting load/patch/ball combo that I am happy with before I file and drift the sights.

Here's another question.  How do you guys sight your rifles that you deer hunt with?  If you use a sight picture that levels the top of the front blade with the top of the rear sight, where is your point of impact at XX yards?  Just curious what you feel works best.

[Daryl]

These are the sights I prefer on a hunting rifle, hense they are on the 14 bore and my .45 longrifle.  Express sights are more difficult to set up as filing on the front sight isn't possible and the rear shallow V must be set up too high, then filed in for a specific load.
With blade and V notch, the sight is taken with the top of the blade even with the top of the rear sight's flat, and the top of the blade intersecting the exact centre of the bull.




Incidently, the express sights are not just for close range shooting. My very best shooting at any range, out to 300 meters, has been done with the same 14 bore rifle pictured. The last bench group of 5 at 200 yards measured 1 1/2" wide by 3 1/2" high.  The last bench group at 100 meters (109yds) was 1 1/2" for 5, a round group.  For aging eyes, this seems to be one of the best sights possible - almost as good as aperture sights, which aren't allowed at BP shoots, yet of course, there are open sights almost peeps that are allowed.  Again, accuracy is very good with these sights - as witnessed at Rendezvous BC 2009, just last month where this rifle took more than it's share of winnings.