Spin Drift

Spin drift is another element that should be accounted for in long range shooting. The effect of spin drift is time based, not distance. You will hear and read that it is not needed to account for spin drift untill you're shooting out to 1000 yards. This is not true. Let's look at what spin drift is first, then how much effect it has on our bullet. Spin drift is a deflection caused by the spinning motion of the bullet. The direction of the deflection is toward the direction of the spin. The amount of deflection depends on a few factors of our bullet and barrel, such as bullet length, flight time, and the spin rate that the barrel puts on our bullet. Let's look at how much spin drift affects our bullet. I'll use two different examples: a .308 and a 300 Blackout. Lets take a look at spin drift for both at 500 yards. At 500 yards, spin drift affects the bullet .1 mil or 1.4 inches. 1.4 inches is not that big of a deal, right? Let's put it into perspective a little bit. I like to shoot a 4 inch target at 500 yards. 1.4 inch is a little over a 1/4 of the size of my target. That's a a good amount. This leaves very little error for a wind call. So, yes, a correction needs to be made. At 500 yards with a 300 Blackout using subsonic ammo, spin drift affects the bullet by .5 mils or 8.5 inches. That is huge. That is definitely the difference between a hit or miss. In these examples, you can see the only thing that really changed is time of flight. The .308 has a time of flight of .68 seconds and the 300 Blackout has a time of flight of 1.49 seconds. As you can see, it's not based on distance, but time. Let's look at the same bullets at 1000 yards. The 308 at 1000 yards has a flight time of 1.69 seconds and spin drift affects the bullet by .2 mils or 8.9 inches. If you are shooting a 1 MOA target at 1000 yards, you better calculate for spin drift, or you will most likely miss. The 300 Blackout has a flight time of 3.15 seconds and 1 mil or 37 inches of spin drift. Now that you know how spin drift affects a bullet's trajectory, let's look at the formula. Drift = 1.25 x (SG + 1.2) x TOF ^ 1.83.  Drift - spin drift in inches, in the direction of rifling twist SG - the gyroscopic stability factor TOF - the bullet's time of fight Example: SG - 1.83 TOF - .7 @ 500 Yards 1.25 (1.83 + 1.2).7^1.83 = 1.97 Inches TOF @ 1000 = 1.75 1.25 x (1.83 + 1.2) x 1.75^1.83 = 10.54 Inches  If you are like me and are bad at math, no problem. You don't need to understand the formula to get the information. But, I do like to have this equation on a formula page in my dope book. The easiest way to get your spin drift calculation is to use a ballistic calculator. Set all windage adjustments like wind, coriolis effect, ect to 0. Turn on only spin drift, and there you have it. This will give you your adjustment for spin drift. This information should go into your tables. I do like to question everything and look at things at different perspectives. While calculating for spin drift is important, if you are a beginner or fairly new to long range shooting, I don't want you to get caught up in spin drift. You need to be focused on the fundamentals. Lets look at spin drift from another angle. At 1000 yards with a 308 and a 10 mph wind, we have a windage adjustment of 2.9 mils or 102.7 inches. If we compare that to a 12 mph wind, we have 3.4 mils of windage adjustment or 123.3 inches. Thats a total of 21 inches of error for a wind call missed by 2 mph of wind. This is a very easy mistake to make. So a wong wind call has a greater error than not calculating for spin drift. This is one of the reasons why I suggest not getting so caught up in spin drift, but focusing on the fundamentals. However, this does not mean not to calculate for it. It's a very easy calculation and should be made for every shot. What I'm saying is, I don't want you to be so caught up in worrying about Coriolis and spin drift if you do not have your fundamentals down. You're more likely to miss a target because of a bad trigger pull or parallax not set correctly, than you are forgetting to adjust for spin drift. The Overwatch