Another question for TR shooters

[karen]

Indeed. They should never ever be added together or compared.
Then there's backlash.
And diffraction.
It's a wonder we ever hit the thing at all really.

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Excellent - more excuses for my bad shooting! :lol:

[Dougan]

And do remember that the "minutes" on your sights are a bit nominal. If you move the rearsight forward 2 inches, that shortens your sight radius by about 6%, which in turn makes your "minutes" 6% bigger. That's a change of a bit over 2 "minutes" at typical 1000 yd settings.

My original question was actually to do adjustments on a scope (which I'm a beginner at), but this is also interesting - My previous TR rifle had a 30" barrel and all adjustment was done on the rear sight; and the raise from 300 to 600 was the standard 10.5 mins...my new TR rifle has a 28" barrel and an adjustable front sight; and the same raise is just under 10 mins...

...so is that the difference in sight length, the adjustment on the front sight, or both...?

[Dougan]

Does anyone know exactly what a 'minute' measures at in inches at 200, 300, 400 and 500 yards?

Mathematically it is 3600*tan(1/60) at 100 yards, which is approximately 1.047" (multiply by 2, 3, 4 & 5 for the specific distances you mention).

In shooting it is very often approximated to 1" at 100 yards.

What you actually get from your rearsight is another thing, and depends on the TPI of the rearsight threads (Alpha1: TPI = threads per inch), and the clicks per revolution, clicks per minute, and the sight radius (between the front and rearsight irises). I once did the maths in order to make my own scorecards with a grid that matched my actual sight movement & radius - the exercise was somewhat pointless, but interesting. I've made a quick spreadsheet that takes the 4 inputs and gives the resulting movement at 100 yards and percentage difference when compared with Mathematical minutes and 'rifle' MOA (1 inch @ 100x). It is initialised with the values for a Trakker with 40 TPI thread and 1/4 minute clicks (from memory). If anyone spots errors let me know and I'll make an update.

http://fbuk.ammara.com/downloads/TR-Rearsight-MOA.zip

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Cheers Rox,

I'm going to have to do some maths revision to understand the first bit...

...but the rest is again interesting - I'm not saying you have to complicate shooting, but some of the detail is intriguing.

[rox]

...so is that the difference in sight length, the adjustment on the front sight, or both...?

The reduced sight radius will probably at least be a contributing factor. You could try entering your current sight radius into my calculator, then reduce the radius by 2" and see how much more the minutes represent. This is only an approximation though; the sight placement on the different rifles could mean that the sight radius difference is not only down to the different barrel lengths.

If part of your adjustment is made up of moving the front-sight then it sounds like you might already be combining/adding front and rearsight minutes to arrive at 'just less than 10 minutes'. Remember that they are incompatible. Some are marked in 6 minute intervals and some in 5 even though they have the same pitch!

I haven't thought about a 'rise' as such for years. I just know that at a various distances my front sight is at A, B or C and the rearsight is at X, Y or Z, and that X, Y and Z are all within a couple of minutes of each other.

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[southern_guy]

The rule is most accurate when the sightbase (distance between front and rear sights) is 36 inches!

[rox]

The rule is most accurate when the sightbase (distance between front and rear sights) is 36 inches!

Which rule is that exactly?

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[southern_guy]

The true minute of 1.047 inches at 100 yards is based on a sight base of 36 inches

[Gaz]

Surely 1MOA subtending 1.047" at 100yds is a mathematical constant?

[rox]

Surely 1MOA subtending 1.047" at 100yds is a mathematical constant?

Indeed. What does work well with a radius of 36 inches is that 0.010" of sight movement equates to precisely 1 inch at 100 yards, which makes it convenient to map a thread with a whole number TPI (Alpha1: TPI = Threads Per Inch) to produce 1 inch of movement per hundred yards - if your sight radius is 36 inches. Which is probably isn't.

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[Dougan]

...so is that the difference in sight length, the adjustment on the front sight, or both...?

The reduced sight radius will probably at least be a contributing factor. You could try entering your current sight radius into my calculator, then reduce the radius by 2" and see how much more the minutes represent. This is only an approximation though; the sight placement on the different rifles could mean that the sight radius difference is not only down to the different barrel lengths.

If part of your adjustment is made up of moving the front-sight then it sounds like you might already be combining/adding front and rearsight minutes to arrive at 'just less than 10 minutes'. Remember that they are incompatible. Some are marked in 6 minute intervals and some in 5 even though they have the same pitch!

I haven't thought about a 'rise' as such for years. I just know that at a various distances my front sight is at A, B or C and the rearsight is at X, Y or Z, and that X, Y and Z are all within a couple of minutes of each other.

..

I measured from aperture to aperture, which was almost bang on 32" (813mm) - Putting this into your spreadsheet increased:

Effect of 1 sight 'minute' @ 100yds - to 1.125"
Difference Vs Math Minute - to 107.2%
Difference Vs 'Rifle MoA' - to 112.5%

So basically, unless you've got a 36" sight base, it's not a true minute, and the shorter the sight base the larger the minute becomes...

...I also see what you mean now about changing the thread on the adjustment.

As for the 'rise' - I got a new rifle last year, and through the season made changes to both the rear and front sights; so never really got a proper zero, and didn't do any plotting - This year I aim to keep everything as is, get a good zero, and start plotting (post shoot) again, which I found really helped understanding how my previous rifle behaved.

I'm not sure if I'll ever shoot accurately enough to see the maths in action...but all this stuff about minutes and inches is definitely worth understanding :cheers: