Transonic Ballistics Effects Explained by Bryan Litz
What happens when the bullet slows to transonic speed, i.e. when the bullet slows to about 1340 feet per second? It is getting close to the speed of sound, close to the sound barrier. That is a
bad place to fly for anything. In particular, for bullets that are spin-stabilized, what the sound barrier does to a bullet (as it flies near Mach 1) is that it has a de-stabilizing effect. The
center of pressure moves forward, and the over-turning moment on the bullet gets greater. You must then ask:
“Is your bullet going to have enough gyroscopic stability to overcome the increasing dynamic instability that’s experienced at transonic speed?”
Some bullets do this better than others. Typically bullets that are shorter and have shallow boat-tail angles will track better through the transonic range. On the contrary, bullets that are longer… can experience a greater range of
pitching and yawing in the transonic range that will depress their ballistic coefficients at that speed to greater or lesser extents depending on the exact conditions of the day. That makes it very hard to predict your trajectory for bullets like that through that speed range.
When you look at transonic effects on stability, you’re looking at reasons to maybe have a super-fast twist rate to stabilize your bullets, because you’re actually getting better performance — you’re getting less drag and more BC from your bullets if they are spinning with a more rigid axis through the transonic flight range because they’ll be experiencing less pitching and yawing in their flight.
The short version: I'm trying to find a round that either can push through the transonic window effectively -- as I have in the past when I had access to 1200 yards -- or, focus on using a round that is effective out to 1000 yards and still supersonic. While my MR762s could do this with the 1:11 twist barrel using Hornady TAP ELD AR 168gr, I am not as confident with a 1:12 twist barrel using Federal GMM SMK 175gr.
And since I cannot test this myself, I was fairly confident that there were SH community members with many years of AW308 experience who could save me the money and trouble. Some of you have answered my question, and I think testing just two factory rounds out as far as I can go is worth it: FGMM SMK 175gr due to it having the greatest stability factor of all the rounds while being heavy enough, and Hornady TAP ELD Precision 168gr due to it having the greatest BC and fastest velocity.
So I'm going to leave this to rest until I can afford to go test these, and will try to find a 1000+ yard range to do so. When I have the data, I will report back.
In the mean time, I'd still like to hear from specifically Arctic Warfare owners on your experiences over the years.
