Australian minutes v’s True minutes

[PeteFox]

I’ve come across some wind estimation tables which give a version in “Australian Minutes” and “True Minutes”.

As far as I knew, a minute was definitive and wasn’t subject to variation.

The two images show very different values for wind at 900m. The upper one is true minutes.

BDC849AE-41FD-4FFA-8450-87C9356BD571.jpeg

4CE919A5-3100-49A4-B55A-66169B8D095C.jpeg

Could anyone please enlighten me.
Pete

[RJNEILSEN]

Yes Peter, a minute of angle is definitive and is one 60th of a degree. This is the movement you will see in a typical telescopic sight that has is designed in moa.

With old school (and new school) target rifle sights, the actual angular movement is determined by the thread pitch in the rear sight and the distance from the rear sight to the front sight ring (sight radius). This is just basic trigonometry and if you know the thread pitch of your rear sight and then measure your sight radius you can accurately calculate how many "true" angular minutes per revolution of your rear sight windage arm.

For historic details on the Australian vs English vs True minutes, we will need some input from some of the more grey haired members of the forum, but I guess it is due to differences in standard thread pitches and barrel lengths of rifles with older English sights vs Australian Central sights at the time.


Cheers,
Ryan.

[PeteFox]

Thanks Ryan,
I’m dusting off my Central sight and I’m going to give TR a go again after 25 years. My scopes are in mils so I had to think about moa again and this had me thrown.
My vernier plates were made when 144gn Factory ammo was the only option and knobs are 1/3 moa per click. I’ll do the measuring and trig to work out what version of moa I have.
Pete

[RJNEILSEN]

Peter, if you have a genuine central sight, the windage thread is probably 1/4"x40 threads per inch. If you measure your sight radius in mm from the rear aperture to the front ring, I can tell you you how many true minutes this is. eg one of my rifles is 965mm sight radius, so each full revolution is 0.75 true minutes. If if I had clicks in thirds, then each click would be around 0.25 true minute clicks.

Ryan.