Thursday, October 16, 2008

Front sight math

Sometimes it's good to know how much area your front sight covers at a given distance and with just a few measurements it's fairly easy to figure. We all know that when we look at something at a distance, it appears smaller. A bullseye that looks large at 25 yards isn't so large at 100 or 200 yards. Also, that front sight tends to cover more area at 200 yards. But how much? Simple.

The first thing we need to do is measure the width of the front sight. For example, on my Handi-rifle with a 22" barrel, the Williams firesight bead I've installed has a measured width of 0.091" (measured with my calipers). I've also determined that when the rifle is shouldered in firing position, the sight is 30" from my eye. So, what I've described is an isosceles triangle, 30" long and 0.091 inches wide. How much width would that sight cover (subtend) at 100 yards? Simple. 100 yards is 3600 inches, and the equation looks something like this:

0.091/30 = X/3600. Solve for X.

So, if my math is correct, that 0.091 front sight, 30 inches from my eye, will hide 10.92 inches at 100 yards. It's good to have that knowledge when I take the rifle afield. Now, how might we use that information?

If a deer's heart/lung area is 18 inches wide, then the front sight will completely cover that eighteen inches at about 150 yards. So, if when I put my rifle to my shoulder, I can see boiler room on either side of the sight, then I'm probably within 150 yards of that animal. Of course, all distance estimates are just that... estimates. Still, it's good to have that little bit of extra knowledge.


George said...

More! More! More!

We can never get enough iron-sight tech info.

Is there a place where this is all written down? Maybe relative to the different kinds of sight combinations?

Boomerator said...

Tell us *why* this little bit of knowledge is useful.

There is something called point blank distance. It is the farthest distance at which you *don't* need to make a bullet drop adjustment to your shot to accurately hit your target.

If for example you know the point blank distance on your rifle is 200 yards (depends on how it is sighted in as well as the load) and you take the same situation the author gives. You are within your point blank range, all you have to do it put the sight on the deer and squeeze the trigger. If on the other hand your sight covers too much of the target you then have to go about figuring the actual distance. That is why it is useful.