range estimation error effects

[PaulStafford]

For fun, I imported the drop in inches from 0 to 2000 yds (data every 25 yards)from JBM into excel. I then used excel's curve fit function to get the derivative, giving me the "<span style="text-decoration: underline">change </span>in drop" vs. "<span style="text-decoration: underline">change </span>in range".

This is instructive in showing how an error in range estimation can affect your accuracy. For my 300wm (230 Berger at 2920 fps), the drop slope is only .8"/yd at 500 yards, but 1.6"/yard at 1000 yards, and a whopping 2.8"/yard at 1500 yards. This means at 1500 yards, a range error of only 5 yards will give you a target error of 14", which might completely miss a 30" high steel plate if you were aiming at the center.

5 yards error in 1500 is only .33%. The Terrapin spec sheet claims an error of 3m at 2400m, which is .13%. .13% range error would generate a target error of 5.5" at 1500 yds in this scenario.

This shows why it can be so hard to use a mil reticle for ranging at 500 yards or so on steel targets. For example, a 24" target estimated at 1.5 mil = 444 yds, but estimated at 1.4 mil = 476 yds.

That 32 yard range error, with the .76"/yd slope at that range gives a 24" target error- a complete miss with only 0.1 mil error!

Your numbers will vary from these depending on your ballistics, of course.

 

[cali_tz]

Re: range estimation error effects

yes and I don't think that the Terrapin error varies with distance either. I did range some targets out around 700-900 over Labor Day witht the Terrapin and got the exact same reading 3 times in a row, but one can't conclude much from that.

The two impressive accomplishments with all this mile shooting stuff is first what percentage of cold bore shots at random distances can one achieve, with the help of a solid LRF like the Terrapin, and secondly for the next n shots, what grouping can be achieved. If I throw enough lead down range, yes I will eventually hit the steel, which is fun at first, but then it's like what's the value if your cold bore percentage is low.

Then on top of your observations, calling the wind right on cold bore, and the on subsequent shots... quite a challenge.