SD goes beyond the rifle twist rate,
The bullet length matters, the amount of spin, the velocity, the point of the trajectory you are measuring it, the wind, not just the wind speed but the wind direction. Actually, the entire bullet design has a factor in it. From nose to tail as well as the groove design.
Most Apps are not "calculating" anything, you need specific data along with a 6DOF to figure out the actual prediction, and those predictions don't include all the above. They just use some basic data and a flat rate value. That value is normally set to equal 1 MOA at 1000 yards with a specific TOF. In other words:
They figure everything that crosses the same point at the same time is drifting the same amount. By doing it this way they feel it helps more than it hurts, and that adding in something is better than adding nothing. I have seen this bit people more so than doing nothing because it changes your wind call.
Get better at calling the wind, and ignore SD.
Some apps you can change your zero range to say, 600 yards and the SD will be the same as if you have a 100 yard zero. We've played with that and demonstrated the flaw in the system
SD is more a buzzword than an actual thing you have to worry about. It gets lost in a lot of noise of the shot, from poor trigger control pulling the shots right (Most people are righthanded, add in a left-handed shooter and you'll see a difference) to blown wind calls, to variations in the bullet vs rifle twist.
I don't know how many times I have to repeat myself, a lot of us never use it and never see a negative effect on our end results. Doesn't get any easier. if it was an actual issue we'd all be using LH Twist Barrels in the USA as a routine to reduce it to a microscopic number. We don't because it really really doesn't matter.
Dope the wind inside 1500m and don't worry about the tiny things.
Here is the deal, don't listen to me, read it yourself, you can find it in more than one book
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