I am just a bit confused as to why the majority of the competition shooters that are shooting 90 vlds are using around 1-7+ twist for the .224 Berger 90 VLDs when Berger's Twist rate stability calculator indicates marginal stability for that twist rate...? I see barrel length doesn't seem to be part of the stability calculator formula does that have something to do with it? Is Bergers calculations based on a particular short barrel length...? That part just has me a bit confused why the pros are choosing the slower twist rate as their preference..? They typically use 30 inch barrels and seem not to be having stability problems.
Paul - the stability is not "marginal". Years ago, when Bryan Litz came out with his first book, "Applied Ballistics for Long-Range Shooting", he advocated using a barrel twist rate that would produce a gyroscopic stability coefficient (Sg) of at least 1.4. Based upon further testing and analysis, he now advocates a twist rate that produce an Sg of at least 1.5. Berger's twist rate calculator utilizes the Sg of 1.5 value in it's determination of how much of the theoretical maximum BC a bullet will retain in a given barrel twist rate, velocity, atmospheric conditions, etc. In these calculations, velocity is critical, barrel length is not, because different loads can produce a wide range of velocities out of any given barrel length.
Shooting a bullet with an Sg of 1.2 doesn't mean it will fall out of the sky. In fact, an Sg of greater than 1.0 (in theory) will keep the bullet stabilized, although I wouldn't try that knowingly. A twist rate that produces an Sg of 1.5 or greater means that the pitch and yaw of the bullet as it exits the bore is minimized, thereby allowing it to "settle down" in the shortest possible distance, resulting in the maximum BC, or lowest drag, the bullet's shape is theoretically capable of.
Stability for a bullet of given length at a given linear velocity requires a given spin rate. Because linear velocity decreases at a much greater rate than rotational velocity (i.e. spin rate), bullets become
more gyroscopically stable the farther they fly. This is because at longer distances they're spinning much faster relative to their linear velocity than they are when they exit the bore. Shooting a bullet out of a slightly slow twist rate that gives an Sg of less than 1.5 doesn't mean it will keyhole at the target. It simply means that the increased yaw/pitch as it exits the bore and during the early part of its trajectory will make it behave as though its BC were slightly lower than the actual value due to the increased drag during that time. What that effectively means is that the bullet will require more elevation to reach the same distance, and be influenced more strongly by the wind, both of which are directly related to the "effective" BC of the bullet. I'm giving up about 2% of the theoretical maximum BC of the 90 VLD by running it out of a 7-twist barrel (Sg = 1.45 at 2850 fps, 1000 ft elevation, 65 degrees F). However, I also point the bullets, which gains back the lost BC plus a tick more. A 2% loss is really not a huge amount, whereas as a loss of 5-7% or more of the theoretical maximum BC would start to become noticeable. That kind of BC loss would be analogous to moving down one weight class of bullet, which most of us would not choose to do.
If you really wish to preserve 100% of the theoretical maximum BC of the 90 VLDs, you can have a 6.8- or 6.7-twist barrel cut. There is nothing wrong with that approach and most of the major barrel manufacturers can cut any twist rate you want. However, the lead time for receiving such a barrel might be as long as 8-12 months. In contrast, there are a number of barrel suppliers that commonly keep 7-twist barrels suitable for F-TR rifles in stock. Further, there have been reports of jacket failures with some of the 90 VLDs. As far as I can tell, the underlying cause(s) of this are not completely clear. However, it is certainly possible that faster twist rates might exacerbate such issues. For that reason, it is not absolutely necessary to use a 6.0- or even 6.5-twist barrel with the 90s. There may not be anything wrong with that approach, but it is not
essential for stability. If you want the full BC without using a faster-than-necessary twist rate, simply special order a 6.8- or 6.7-twist barrel and then wait for it to come in. Otherwise, it is much more likely that you will find a suitable 7-twist barrel in stock somewhere that you can have in-hand within days, rather than months.