Help with "sight adjustment math"

[Silvertp]

I just purchased a  mauser 280 AI that was put together by several different gunsmiths; stocked and accessorized by one, barreled by another and chambered by a third.  While fireforming some cases I decided to sight in the "open sights" before I mounted a scope as I purchased QD rings in the event I wanted to swicth back and forth from scope to open sights.  Informal plinking I found that after taking all the elevation adjustment possible with my rear sight I was still hitting considerably high, so I decided to head for the range to get a more accurate assessment of how much off the sights were at a measured yardage.

My group center is 18 inches high at 50 yards.  I cannot lower the rear sight any more.  I realize this means that I need either a taller front sight, or a lower rear sight.  I would much prefer a lower rear sight, and the rear sight visibly appears to be mounted very high above the bore, relative to the front sight.  The rear sight has some small script that appears to say ERA.  Distance between the front and rear sights is 14 5/8 inches.   

So now my question, how much lower will the rear sight have to be to appropriately sight this rifle in?  Hope someone out there remembers more of their high school math than I do.

Silvertp

[LaOtto222]

OK, here we go...convert everything to feet to start with, then back to inches. 14.650" = 1.220'; 50 yards = 150'; 18" = 1.5'. Now we do a ratio to find how much higher the front sight or lower the rear sight needs to be. 1.5"/150' = X/1.220" then (1.220 X 1.5)/150' = X  so 1.83"/150' = X; X= .0122' convert to inches .0122' X 12" = .1464 inches. 1/8" = .125"; 3/16" = .1875" just for reference. This is the amount you have to lower the rear sight or raise the front sight OR a combination of the two. This should get you dead center to the aim point at 50 yards. of course you may have to make more adjustments to get it dead center at 100 yards, but more than likely you will be OK, shooting low at 100 yards and then can adjust it up. 

[LaOtto222]

I would like to add just a little more after some more thought. The above assumes you are going to always be shooting the same exact load. If you change velocity or bullet weights or what ever, you will change the bullet impact. If I were you I would go for 3/16" to make sure you can have enough adjustment for different loads. Just my 2 cents. Good luck and let us know how it works

[Silvertp]

Thanks LaOtto222....That is exactly the help I was looking for.  I'll have to do a little research to see if the the ERA helps me find the sight manufacturer and find out what other sizes they may have available. 

In looking at the height of the base the rear sight is mounted on, the bottom of dovetail is 3/8" above the top of the barrel, so I should have  plenty to work with.  Good point on the effect of changing loads on POI, as I have no idea what load(s) I will end up shooting.  All I know for sure is the sight combo on the rifle at this time requires a little more kentucky windage than will work for me.   

I'll let you know how it all works out.

Silvertp

[Rangr44]

You can use Brownell's Sight Height Calculator to automatically figure what you need - Just plug in the missing measurements.

http://www.brownells.com/aspx/NS/GunTech/sight.aspx

[Silvertp]

Thanks Ranger 44. 
Thats the kind of math that even I can do.  Ive visited the Brownells website several times and never knew the "sight math" page even existed.

Also, I found out that the rear sight I have is made by NECG (New England Custom Gun).  I called them this morning and am working with them to come up with a solution to my sighting-in woes.

Silvertp

[LaOtto222]

I just went to the Brownell's site for calculating sight height adjustments. Plugged in the numbers and came up with the same thing I had done. I used 1800 instead of 3600 (50 yards not 100 yards) I did it just to check if I did it correctly. They converted every thing to inches , which is less confusing than converting to feet I suppose. It was the same formula...a simple ratio calculation.

[ncsurveyor]

I'd readjust your rear sight to the middle of its range first.  Than shoot again and see how high you are. 

I'd rather have the most flexibility in adjustment for the future.

[Rangr44]

You're welcome, Silvertp - FYI, that type of sight math is called the K.I.S.S. method.