Re: Post Transonic-Sub MOA Accuracy
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Phil1</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Mr. Humble</div><div class="ubbcode-body">Lots of heavy math needed. A 300 Whisper can do that kind of accuracy at 1K and it's subsonic right now. 6.5x284 will blow off a 308 at any range, so maybe it's more about SD than velocity. </div></div>
But with the whisper, many are the loads are entirely subsonic.
My intent with this thread is to discuss the "myth" or is it a physical fact backed by empirical study. That bullets become more unstable subsequent to the transition to subsonic flight. That as such, the accurate range of any bullet, or certain bullets(and calibers)should be limited to the distances where the bullet transitions to subsonic flight.To maintain that MOA or less standard.
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Here's the quick and dirty as I've seen it. Some bullets are more susceptable to becoming radically unstable when approaching the transonic range. Some become unstable but not so radical. Some it doesn't seem to affect.
These are the factors I've seen that affect stability:
Altitude- The higher the altitude you are shooting the thinner the air and less effect on the bullet.
Temperature- The higher the temperature I've found the less bullets get affected.
Condition of air/air density- Humidity and high density altitude (hot days typically show higher altitudes {density altitude is given like altitude; high altitude=high density altitude}) A humid day acts like a hot day. Low number of hg's means bullet flies better. High less so.
Example: shooting here in MN in the summer it isn't that hard to get a 168 CC or SMK to get to 1k without key-holing. Temps usually run about 80-84 deg. and humidity here is 80-90% Actual altitude is 1000 ft. The bullet enters the transonic range around 890 yds. In the summertime they stay stable. But, in the winter when the temp is 40 deg. or below you have a tough time keeping them stable out to even 900. At -18F. ...well you can imagine.
Barrel twist: The tighter the twist the better I've seen bullets remain stable. No matter what bullet is used and all other factors. Twisting a bullet faster, without destroying it internally, promotes better stability. That is as long as the bullet is completely concentric. A non-concentric bullet will become less stable from the get go because it wants to revolve around the barrel axis instead of it's own axis.
Bullet shape:
Shorter fatter bullets do tend to remain more stable through the transonic range. The 168 CC and SMK are examples of bullets that get disrupted, but restabilize well after the transonic disruption. They do lose a certain amount of accuracy through the disruption, though. But, all bullets lose that to some degree.
Longer sleeker VLD/ULD type bullets lose the most stability going down through the transonic range. These are the ones that often-times become radically unstable(never recover after becoming unstable). The way to beat that with these is to step up the twist. But, then you have a whole new set of factors as the bullet leaves the barrel. If you go with the minimum twist that will stabilize any VLD/ULD type bullet they will be very accurate out to the point of instability. You just have to accept they don't always work past that. Add in other factors though and they may not become radically unstable. i.e. shooting VLD's in the high desert can be as accurate as far as you can see. Send a Berger 7mm 168 out in sub-zero from a 1-10" twist barrel and it will be unstable when it slows to the transonic range.
There are those bullets that tend to remain more stable by incorporating the best of both worlds. Long ogives, but with compound factors in them, i.e the Berger 175 Hybrid and 175 SMK. The 7mm 168 SMK doesn't have multiple ogive factors but it remains pretty stable from a 1-10" twisted barrel well past the trans-sonic range. Lapua Scenars are about the best example of a really good high BC, but still short enough and smooth enough that the transition through the transonic range doesn't affect them nearly as much.
The transonic range is important and does have an effect on bullets because the 'sonic-boom' aka 'snap' you hear, is a force that applies itself when it catches up to the bullet. It physically moves the bullet. Which when stable and flying along is disrupted. The disruption causes a yaw with the bullet. Two forces then counteract each other. The yaw of the bullet wants to make it move at a 90 degree angle to the point from which it touched the bullet. The air coming at the bullet wants to exacerbate (increase) that movement. The rotational spin (gyroscopic force) wants to make the bullet spin around it's original axis, and therefore settle back down. By increasing twist you provide more force at the end of the bullets journey to do that. So, the battle is between those forces. The bullet may not settle back down (radical) or take it's time, or go right back to where it was without hardly seeming affected. Each of the factors increases or decreases with each variable you change.
