Range Report Transonic range vs. bullet stability

[turbopromod]

Hi all, new to The Hide, first post. Great site!

As it relates to accuracy, does a projectile usually become unstable just at the point that it crosses through the transonic region, or does degradation in flight path usually take place at some point after going from sonic to subsonic? How do projectile parameters affect the stability of a bullet as it goes through this phase?

I am very new to shooting rifles, and have a real ineterst in long distance shooting in particular; I have been shooting a 10/22 at 50 and 100 yards to work on all my basic marksmanship skills. In the process of doing this, I discovered something sort of odd. While my particular rifle shoots well (.5 to .75 MOA very consistently) at 50 yards with subsonic ammo such as Eley or Remington Subsonic (1070 fps or so, 38gr), 100 yards with this ammo is a joke. It doesn't follow the trajectory curve at all, and is all over the place windage-wise. When I step up the ammo to a CCI MiniMag (1235 fps 40 gr), I notice that the 100 yd groups come right on into where they should be (1.5-2.0 MOA or so). What's getting my attention is that, according to my ballistics calculator, the MiniMag goes subsonic somewhere around 30-40 yds or so. If this is true, then shouldn't I be seeing a lot more variability in my groups at 100 yds than what I am? This would lead me to believe that, with this particular round (the MiniMag), that the instability is actually introduced somewhere well after the projectile goes subsonic.

Do all rounds behave this way? Is there a way to consider BC and projectile shape and make a prediction of where this instability is going to take place? Am I missing something (more than likely)?

I have a R700 in .308 that I will be stepping up to shortly, and I'm trying to figure out as much as I can now so that I can understand and apply what I'm learning with the .22LR.
Thanks for any and all input!!

 

[Greg Langelius *]

Re: Transonic range vs. bullet stability

It's a good question, with not a lot of certainty for an answer.

A lot depends on the degree of gyroscopic stability the bullet possesses, its aerodynamic stability, and just plain random luck. Some bullets will bore through the transsonic with minimal diversion, some will get a lot.

In essence, as the bullet slows through the transsonic region, the shockwave migrates back along its length, and this can disrupt its angle of attack, causing to to veer and divert. So whether it happens starting just above sonic, or just below can be unclear, but beyond some part of that transition, its path diverts.

As a simplification/rule of thumb, it's generally wise to keep bullet speeds at somewhere above 1200fps (1300 is better) at the target distance; and if that's not feasible, to expect the cone of despersion to broaden beyond that velocity regime.

If the chambering/bullet combination won't do that, you're stretching it beyond its natural capability.

Efforts to drive projectiles faster than that at the target are not especially productive; nor is hotloading a marginal bullet/chambering combination to force it into the proper velocity range. What one may gain in performance, one loses proportionally in barrel life.

I favor barrel life over performance, and choose my chamberings accordingly; like .260 over .308, and .280 over .30-'06. I alse prefer to use the lighter end of the suitible bullet weight range (I prefer a 150gr o/o a .280 over a 175gr o/o a .30-'06), and to substitute winds skills for overpowering energies. Finally, additional recoil stresses the shooter more, adding to fatigue over a sustained course of fire. There's not a lot of point in making the process harder.

One of our members, Brian Litz, has more practical and mathematical experience with this question. Perhaps he can add some clarity here.

Greg