Re: 50 BMG goes subsonic at 1500 Yards? Effect?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ELR researcher</div><div class="ubbcode-body">...What is the effect - from the standpoints of accuracy and stability - of going subsonic? </div></div>
Going subsonic is not the problem. Going <span style="text-decoration: underline">tran</span>sonic is the problem.
As it slows, a supersonic bullet does not go subsonic all at once. Somewhere about 1.2x SoS, airflow over small portions of the bullet will begin to lose SoS. The more it slows, the more of the bullet that loses SoS until finally there is no remaining supersonic flow (which usually happens about 0.8x SoS). It's only then that the bullet is <span style="text-decoration: underline">sub</span>sonic.
The reason transonic flight tends to be problematic is because of "wave drag." Wave drag is a consequence of reaching the speed of sound and is much stronger than the forms of drag a subsonic bullet experiences. But during transonic flight, only parts of the bullet are experiencing wave drag. Such a strong force acting irregularly on the bullet will tend to upset its stability.
For the longest time, the speed of sound was the bogey man for both aviation and ballistics. The transonic region was a sort of "no man's land" to both fields, except that airplanes first needed to accelerate past it without flying to pieces while bullets needed to slow through it without meeting a similar fate.
Historically, maximum range was calculated as the greatest distance a given bullet launched at a given velocity could be expected to remain supersonic because most bullets became unstable when they backed into this transonic region. Which is why discussions like this often focus on the whole supersonic/subsonic thing, but that's in the process of changing. Some bullets now are noted for being able to slow through the transonic region without losing stability. Probably the most notable is the 250-gr .338 Lapua Lockbase (<span style="text-decoration: underline">not</span> the Lapua Scenar, which was used in the CheyTac tests, and which has a different construction). The Lockbase was the bullet used by
a British sniper to break the "world record" sniper shot, at a distance something like 800 yards further than his rifle's "maximum range" as determined buy the old-school speed of sound method. That could have been a freak shot, pure blind luck, except that once he'd got the range, he shot two Taliban on successive shots (or so the story goes). But if it's now possible to regularly score hits beyond SoS, then we've entered a whole new era.
I don't have the expertise to take a side in the turf war over the Cheyenne Tactical testing other than to note that it took place in 2001. AFAIK, it's only been in the last ten years or so that certain bullets began demonstrating the ability to survive the transonic transition with their dignity intact. I think a lot of the focus in that regard is being driven by the fact that the war in Afghanistan is proving to be sort of the golden age of sniping, lots of engagements beyond 1000 yards and even beyond 2000 yards is not unheard of. If you remove the speed of sound as a barrier, all that remains is having sufficient bullet velocity to produce the desired effect on the target.
Anyway, that's my answer to your question about the effects on accuracy and stability.
It bears noting that Berger Bullet's ace ballistician and regular poster to this forum, Bryan Litz, has designed a 175-grain 30-caliber bullet to remain stable through the transonic region, and is selling a .308 Win tactical ammunition built around it through his company,
Applied Ballistics.
