Rossi Rifleman

I've been working on designing my own rear sight for my R92, and if it works I'd be offering it to the general public. I'd like to be able to offer it with correct "click" MOA adjustments, at least for elevation. I'm making a huge assumption, and wanted to check here to see if anyone can confirm or counter it before proceeding too far.

I'm assuming that if I take the front sight as the fulcrum point, and take the distance from the front sight to the rear sight as DS, the distance from the front sight to the target as DT, the elevation change of the rear sight as ES, and the impact elevation change as ET, I should be able to calculate the needed change in rear sight height to achieve a 1MOA change in impact with the formula DS/ES = DT/ET. See the attached diagram for (maybe) a clearer idea of what I'm trying to say.

What I'm getting at, is that, even though the bullet does not move in a straight line, the drop should be constant to the point that I can do the math as though it does move in a straight line, and calculate the thread pitch that will give me "clicks" that equal 1" @ 100yds.

Am I right or is it just not this predictable?

I'm sure there are people here on the board that know more about engineering, math and ballistics than I do. What I can tell you is, even the major receiver/tang sight manufacturers struggle with the same problem. The problem is further complicated by barrel length variations which effect how much difference one click of adjustment makes. One click of windage adjustment makes a larger change at 100 yards on a 16" barrel than on a 24". I use Williams Foolproof receiver sights on several of my guns and I don't worry about whether the 1/4 minute clicks are really 1/4 minute or not. It would be nice if they were, but they are not. I use mine for Cowboy Lever Action Silhouette and I sight mine for 50, 75 and 100 meters, write down how many clicks up or down for each range and carry my notes to the range. I would try to make sure that each click is consistent and repeatable, and let the shooter figure out what changes are needed to adjust for the range. Just my .02 cents.

Yep, rman is right. Unlike a scope where the distance inside the scope tube never changes, unless you design a different sight for the different possible barrel lengths, it's going to be like a broken watch. At least it's right twice a day. LOL

As a shooter, I'd prefer the threads be as fine as you can cut them with or without indents to give finer adjustment no matter how much it moves it. I like having the fine adjustment that scopes have. Problem is, even the best steel sights I've had never were that fine. But they were at least functional for as accurate as I am with open sights. ;~)

As to the thread calculations. I'd have to think about that and it's just too late and I'm just to tired. Getting old, but I'll sleep on it or dig out my old Machiner's handbook if someone else doesn't chime in with the formula for ya. I THINK you're on the right track, but it's been a long day and my brain is hurting. I'll get back with ya tomorrow. ;~)

Part of the problem with fine threads without detents is that the recoil impulse generally will loosen them.

I have a 'BoMar style' sight on one of my 1911s that had a set screw to lock the adjustments in place.
I had to tighten that screw EVERY range trip. Blue thread locker it. AND STILL had to replace it at least once as it backed out under recoil.

I'm all for fine adjustments but I'll trade that for a reliably rugged repeatable adjustment.

I would be planning to make different models for each barrel length. With the design I'm contemplating it would only be two small components that would be different.

Sounds like a plan. Let us know how it works out - photos, range report, etc.

aragornelessar86 wrote:I would be planning to make different models for each barrel length. With the design I'm contemplating it would only be two small components that would be different.

Well, after thinking about it, I was right...YOU were right aragon. LOL

It's a straight forward ratio calculation. Of course you already figured that out. But you did leave out on variable in your calculation that was throwing me for a loop. I would label the distance from the front sight to the center of the bore, FS. It's actually the same distance as ET, but it is the fixed fulcrum point for the calculation and the ET thing was going right over my head. LOL

I think what you really need to know is what portion of 100 yards, DS represents. In other words, DS in inches divided by 3600 inches. Then you can multiply 1" adjusted POI by that number to get the ES POA to move POI one inch out at 100 yards. I THINK???

In other words, DS in inches/3600 x 1 = ES, I think. Then use that number to do your calculations for threads per inch to get the pitch.

But I really wish someone who still uses their math skills would chime in. I'm old and crusty! LOL

Oh, and since it seems like you are thinking this through pretty carefully, I'll just remind you of the obvious since that's the thing I most often miss. Be sure you don't mix units of measurement when you do your calculations. As in 18 inches and 100 yards. I'm an old dude and I still do it all the time. Makes me cuss and say things every time! ;~)

And yep, I'm with rman. I look forward to seeing what you come up with. There was that rear sight that fit in the safety hole that is really ingenious. I can't wait to see what genius gadget shows up next. I thought I was pretty good, but some of the guys on here have raised redneck engineering to an art form! ;~)

JDB, I spent 4 days wondering why I just didn't feel right about my numbers the first time I calculated them, and I realized that I had mixed inches and yards. I need to double check at some point, but in the end I calculated that a 32 thread pitch would give me 1"/click @ 100yds with a 20in barrel, which is what I currently have, if there are 5 clicks/revolution of the adjustment ring.

I give you the Mars lander.
TWO different divisions of LM one using METERS the other using FEET and some programmer/engineer who didn't bother or didn't notice the difference smashing the vehicle for lack of deploying the landing/recovery system at the right altitude.