1888 Trapdoor

[ssgross]

I don't want to go to the range with a file in hand to set my new front sight height, and certainly don't want to waste ammo getting it just so. Instead, here is a little math.

1) Set your rear peep to where you want it. I set the arrow at the bottom of the triangle of the buffington sight at the 300 yd mark. This puts the peep somewhere below the 200 mark, with just a tad of room left to adjust down if needed before the peep gets obstructed.

2)We need the height P of the peep above the bore axis. OD radius of the barrel at the rear of the sight base is ~ 1.000/2, the thickness of the sight base is 0.115, and I have the peep at 0.275 above that. Add all these up to get P.

3) use your favorite ballistics calculator, with sight height set to 0, weight velocity and BC of whatever bullet you shoot, and the zero of whatever distance you want. I want 100yds and the later-original 500gr round nose at a mild ~1200 fps. If you want the 200 or 300 marks on the rear sight to be exact for your load, set it there, calculate P, and set your calculator with zero at that distance. The drop (in MOA) at 1yd gives you a very close approximation to the launch angle, which is the same as the angle you need between the peep and front sight tip. Close-nuff-fer-govt-werk. Call this angle alpha. If you want alpha exactly, you will need some calculus to get the 100 yard drop with zero-distance set to zero. Fortunately, my calculator lets me use 0.0001 yards as my zero distance.

4) Let h(P) be the height of the front sight above the bore axis needed when the rear peep is at P. Drawing a picture, it's easy to see that
h(P) = P-R*tan(alpha)
where R is the sight radius (21+5/8 on my rifle). Don't forget to convert alpha from MOA to radians or degrees for whatever calculator you are using!

5) Subtract off the OD radius at the muzzle to get the blade height needed. In my case, I need 0.432 for the 500gr round nose, and 0.431 for the 405 gr hollow base. Don't fuss too much, as likely the cast bullet BC's online you found were all over the place, and you made your best guess, and notice that the extreme spread of blade calculated blade heights for these BC's was maybe +/- 3thous. You're just trying to get on paper, then we can use that data point to dial in anything left to do.

Now, the square blade I got from SSfirearms is 0.412. If I run the calculation again with an extra 3MOA for a 6'0clock hold, I get pretty close enough to what I have without needing to shorten really. We are close enough to be on paper. All I need to do then is slope the blade forward to the profile I like. The original blade that bubba had soldered in place was exactly 0.445. He must have known what he was doing, or got lucky experimentally

[ssgross]

Very happy how this turned out. We'll shoot it, and decide if we want to re-cut the crown to fix the heavy cleaning rod wear. I'm guessing nothing spectacular or outright abysmal first time out. That wear is pretty bad, and uneven.

[browningautorifle]

Nice...