Why supersonic?

[Natty Bumpo]

I've only been hanging out here a bit more than a month; but, I occasionally see reference to the importance of a bullet staying supersonic at a given range. Why is this important? Does something happen to bullet stability or performance when it drops to subsonic levels?

I was thinking the speed of sound was around 600 miles/hour. If my math is correct, this works out to around 880 feet/second. Is this close?

[landcbeitner]

Yes, when a bullet makes the supersonic-subsonic transition (transonic zone) it's deflected from its origional course. The 416gr Chey Tac bullets are supposed to minimize this problem with advanced bullet design (called their "Ballanced flight process". If you want accurcy a supersonic bullet must remain supersonic. Trying to shoot .22's you may notice a major loss of accuracy at a give distance, due to the bullet going subsonic. To combat this many will shoot subsonic .22 ammo, you'll have more drob but the bullet never has to encounter the "transonic zone".

[phorwath]

The speed of sound at sea level is about 1100 ft/sec.

Bullet speed is also important in long range hunting in that many expanding bullets start to fail to expand as bullet speed drops below approximately 1700-1800 ft/sec. If they fail to expand then they become less lethal and the chances of wounding an animal increase.

[britz]

+1 on landcbeitner

Whenever someone mentions the fact of staying supersonic, they are mostlikely talking about accuracy. Since bullets don't usually have enough killing power left to take big game when they get down to the 1100 fps mark anyway, the bullet expansion theory is pretty much a moot point. IMHO Varmints can be safely taken with bullets going less than the speed of sound because they are much lighter skinned.

[phorwath]

If the question was with respect to long range hunting, then staying supersonic is more or less the moot point because bullets lose their ability to expand at velocities which are substantially higher than the speed of sound. By the time the bullet has fallen to supersonic velocities, most people would agree that we should no longer be targeting game animals as the bullet will no longer expand, and kill game, reliably... shy of a head or central nervous system hit. If the question is with respect to shooting targets, then supersonic bullet stability is indeed the primary point of this thread. And the third possibility is that Natty Bump was interested in all applicable perspectives. I think we've provided the pertinent information for all apparent possibilities.

[James Jones]

Bullet desgine has more to do with the bullet "destabilizing" than the speed , example the 308 shooting a 168gr Sierra Match King out of a 1-12 twist barrel at 2700fps , this bullet will go subsonic between 800 and 1200yds and the vast majority of the time it s accurcat will fall of as the bullet with start to tumble. shoot the same bullet out of a 1-10 twist barrel and the "destabilization zone" is differant.

Now a 308 shooting a 220gr Sierra Match King out of a 1-10 twist barrel at 2400 fps , it will go transsonic at around the same range maybe a little farther or closer yet this round has been used to shoot to 2000yds despite the bullet being well below the speed of sound. Is it a matter of RPM's? , can't be because the 220gr bullet is starting out slower so the RPM's will be lower (compairing the 1-10 and 168gr bullet).
from what i have seen bullets with BC's in the .525 or more in calibers from 30 and down will stay staybilized after they drop below the speed of sound because of their superior desgine and high BC.

[sullijr]

If you have watched any of the manned space flights on tv they have talked about max Q which is a time during liftoff when communication is lost and the vehicle is submitted to buffeting,I believe that this is when the speed is passing through the "sound barrier" or going supersonic.This max Q also happens when it slows down.A bullet is also subjected to this buffeting, thus a possible loss of accuracy if the bullet is subsonic when reaching the intended target.