Advice on calculating Hold-over/under mil dot distances

[bullbarrel]

For example my TX200 holdovers wouldn't match up with anything chairgun said UNLESS the ballistic coefficient of the pellet was changed. That led to some googling which is when I found out the ballistic coefficient of the pellet is determined by the power plant (spring/PCP/CO2) and to a lesser extent the individual barrel on your airgun. So the default values don't always work.

I'm sure Jim Tyler did an article about changes to BC of the same pellet. Velocity can also have an effect (at least it does with fullbore) so the BC can change as pellets slow down at longer distances.

[Roooss]

Thanks everybody. This is all superb information for me to start digesting.

I believe I need to go away and understand what ballistic co-efficient actually is and how it can vary before I get deeper on holdovers and the like. That said, I will have a play with the Hawke Chairgun app now....

Rifleman's Firearms sounds like a location directly form heaven! smoked meats and rifles! only an hour away from me too so will have to check it out pretty soon.

I am a note taker, so if there is interest, I could write up my learning and experiments after I get down the range and do this.

[Steyr]

The link to Anston is very good. It loos like the work of Brian Samson.

Mil dot is pretty rubbish for calculating range as it is really a military system based on the average man height.

It is a good idea to get a load of A4 paper print outs with a large vertical cross + and put them out at set ranges from 8, 9, 10, 11, 12, 15 20 25 30 35 40 45 50 and 55 yards.

On a piece of paper, draw your ret with the + and the mil dots.

Shoot the target with the scopes cross hair on the centre for each range and you will see how the pellet stike changes as you go from 8 yards to 55.

Just mark the distance on your piece of paper.

Redo this onto some paper or waterproof material with all you POI's marked.

Mildots will only work when the magnification is set just right - typically 10 x but not always. This is where the print off from Anston helps as it will allow you to adjust your mag ring to the ACTUAL calibrated mag where the mildot scale is correct and not what the mag ring claims it is...the two can be very different.

If your doing HFT set the focus so that 40 yards is JUST slightly out of focus and that 45 is even more so.
You will also have a similar crossover point closer too say 15Yards. That way, if the image is slightly or very blurry, you know if your target is Under say 15 or 40 yards or further.

Higher mags narrow this depth of field and there is a balance as to how much mag you need Vs how blurred and unhelpful the image is.

If its for FT, then you will probably be using a very high mag scope and mil dots are pretty much for me.

You really want a fine ret with mil an sub mil substensions for more accurate placement. MTC's SCB ret is a really good ret for air rifles either HFT/FT or hunting and much copied.

I have shot comps and hunted with a 30/30 ret, mil dots and everything inbetween, but the SCB ret is a great favourite and does its job very well indeed IMHO.

I would not go back to Mildot.