Speed Drop Factor, long post.

Shootin Stuff

Sergeant of the Hide
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Feb 11, 2020
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I just could not stop thinking about @jackmaster article on Speed Drop Factor and how incredibly useful and versatile it could be in real-world situations, so I sat down and drew up a few tables and then went shooting.

I first started thinking a +-mil range is all well and good for targets that scale with range, but what if your target doesn't scale with range? Deer, hogs or blue helmets don't get larger the further away they get.

Once drawing up a table for a 2,500ft Density Altitude with factors of 2.6, 2.5 and 2.4 I then added a linear displacement from waterline using the various Mil values it produced at range.

The table itself is based on a 7" vital zone diameter, or +-3.5" from waterline. As you can see, 0.25mil for example at 325 vs 600 vs 850 give vastly different linear displacement values.

SDF comparison 2,500ft DA.jpg


It became obvious very quickly that 2.4 was way too low (and as I found, the ideal number increases with an increase in Density Altitude) and after these tables were created from 500ft to 5,500ft Density Altitudes, a SDF of 2.6 was my chosen value.

I'm still in the process of cleaning up my dope table to make it faster to navigate through, but here it is covering anywhere within a range from 100-1300yds and DA of 500-5,500ft

2.6 SDF dope card front back.jpg


The RED numbers fall outside of my +-3.5" zone if yardage is dialled without applying the listed correction, the GREEN numbers fall within my zone with no correction needed, but it is listed for fine correction if needed.
The numerical values are a correction (either holding under/over or dialling a correction on the turret).

I took this info out to the range yesterday and could not believe just how fast and easy this method is to use in the field. With a spotter calling yardage for various targets all I had to do was dial to it and watch the steel swing.

AAAAAAANNNDD that's when the penny dropped...
 
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Shootin Stuff

Sergeant of the Hide
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Feb 11, 2020
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The penny I am talking about is a range deviation from a set position, rather than a TOTAL yardage

Say for example you are on a small ridgeline over nice flat desert with a mud hut 480yds away.

If you could ZERO your range finder to that 480yd mud but, it would read a range deviation +- from that mud hut, rather than total yardage for any targets within the vicinity.

You then dial 480yds +-correction factors and thats when the magic happens.

Your spotters calls will be EQUAL to your hold over/under and allow for extremely quick and precise shots to be taken at various ranges with no correction needed.

EG a target at a total of 560yds is read by your spotter as plus 80, you hold 0.8 over and make a hit. Your spotter then calls minus 120 (target at 360yds), you hold 1.2 under and let it rip. ETC ETC ETC.

Maybe I'm just a troglodyte, but I had never even heard of this method until 3 days ago and it has already completely transformed my way of thinking about elevation data management.

We tried this with pre calculated +-yardages from known objects and it was stupid simple and rediculously fast.

Now the only thing left is if anyone makes a zeroable LRF?
 
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TheOtherAndrew

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Jan 27, 2021
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What scope are you using?

Sounds like you set the turret zero to your SDF - what do you do for more precision past 800yd - or a target inside 150yd (aka do you still dial or is the scope now a dedicated SDF scope)?
 
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Barelstroker

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  • Sep 11, 2019
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    I haven't tested it yet but my ballistic solver tells a promising story.
    For my 243 Win, using 87grn Hornady Vmax @ 2980 ft/s, I've worked out a drop factor of 1.6 which combined with a realistic point blank radius of error will take me from 200 to 660 meters. I think this will work best for my purpose if I adjust the turret to -1.6 below the 100 meter zero & use the reticle.
    Can't wait to try this system out.