Bisley Field Target Organisation
Rangefinding Test
The difficulties with bracketing are that it's easy to miscount the number of increments - especially when you're in the middle of a competition when the pressure is on. It's also not always easy to keep the rifle perfectly still while taking a measurement and a difference of even 0.2 of a mildot is enough to put you waaaay off on range.

Here's a test to help illustrate some of the difficulties you'll face when trying to bracket a target. Your browser will need to support FLASH in order for this test to work.

Measurements are taken from the Top of the hinge plate to the centre of the killzone, when you have decided how many mil-dots it is, there's a range card next to each example for you to read off the range from - see how accurate you are.

Hint - the hinge plate is about an inch higher than the baseplate.. it's clear to see on some targets, not so clear on others, but these are real photo's of targets from a real course - so this should help to illustrate that bracketing isn't as simple as it seems.

 
Something that the test doesn't illustrate and it's probably the biggest cause of inaccuracies is this...

The human eye/brain can concentrate on one thing at a time (or maybe that's just me?) so for example you might decide to use the thick part of the bottom post of your ret as the starting point for your measurements.. you concentrate on aligning the post with the top of the hinge then start looking up the ret to take a measurement. As you do this you'll find that you naturally start lifting your rifle up as well - giving you an incorrect measurement. You really need to be able to concentrate on both the start point and the end point at the same time and make sure your gun is perfectly still throughout. It's alot easier to do with a mouse on a flash demonstration than it is in real life on a course.

You only have to be out by a fraction of a mildot and you're far enough out on the range to miss the kill.