Bullet trace (what gives?) visible then not...

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DandyBoy

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I need someone to explain the science of this to me, or just an explanation in general terms.

My friend and I, we're doing some shooting @ 500m today with bullet trace / vapor being totally visible all the way to the target with his rifle. When we switched to my rifle bullet trace is totally invisible.

I always thought that if the conditons were right that trace was visible, it would be visible no matter what your shooting. Is it a caliber thing? Bullet speed thing? Twist rate or bullet weight thing?

Really no idea here

Specifics are;

Visible trace-
28" 1:14 twist
308 win with 180g @ 2450fps

Invisible trace
24" 1:10 twist
30-06 with 165g @ 2850fps


Conditions;
-1 with 2-3 km/h winds
Snow covered back round
Black target
 
Where you looking at each bullet path in the same orientation?

If you are shooting side by side and you are looking at his trace and he is looking at yours, you may not looking at the same orientation relative to sunlight.

Best is to set up the spotter and firing point in the same place and just switch... that way, you are viewing the same path to the target and should see the same trace. Now as the sun and air changes, the trace may change intensity or just dissappear.

Jerry
 
Mystic Precision said:
Where you looking at each bullet path in the same orientation?

If you are shooting side by side and you are looking at his trace and he is looking at yours, you may not looking at the same orientation relative to sunlight.

Best is to set up the spotter and firing point in the same place and just switch... that way, you are viewing the same path to the target and should see the same trace. Now as the sun and air changes, the trace may change intensity or just dissappear.

Jerry
Click to expand...

Agreed. And when the sun is behind you as well. Atmospherics and existing wind/mirage can make the traces less visible in some cases.
 
I would say it was shooter/spotter position after the change over. If you can see it with a super sonic .308 you should see it with all supersonic .308s.

I also call it "Swirl", I know some US shooters who call it "Sworl" while trace is used only with tracer ammo.
 
I'm no scientist but on some occasion, I could clearly see a bullet trace and when changing position, (from prone to kneeled) I could no loner see the trace. That's using the same rifle with same ammo on the same day, just a few minutes apart.
 
I think parallax adjustment also comes into play. You'll see the swirl better if your parallax is set a little short of the target distance.
 
To answer some questions so far,

We were spotting for each other with our rifle scopes. Both scopes were of good quality and more importantly both scopes were at 9x magnification.

Our positions were right beside each other, maybe 24" apart.

Sun was at our backs.

It's weird I would shoot my 30-06 no swirl
He would shoot his 308 win and get swirl
We switch rifles and stayed in the same spot.
I shot his 308 win and got swirl
He shot my 30-06 and no swirl.

Very confusing.

Maybe remington 700 30-06 are swirl proof.��

Extra feature I guess
 
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Looking at your set ups there would be a noticeable difference in trajectory. Could be possible you werent looking in the righ spot or the arcy trajectory of the .308 was putting it into more favourable lighting conditions.
 
Bingo. First thing I thought of. You were looking in the wrong spot. When looking for swirl don't look at the tgt. Relax your eye to an area (depending on range) just slightly above the tgt. Have your shooter tell you when he is about to fire so the spotter doesn't blink.
 
In addition, I think that swirl is more likely to be consistently seen through a spotting scope set up directly behind and just above the shooter's rifle, rather than through another rifle scope beside the shooter.
 
I find it interesting that your tracers are different weight they are the same weight for my 30-06 and 308/7.62x51 you using custom made or retail/factory rnds?

Or are we speaking about the disruption of air when the rnd travels
 
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I don't believe that every round fired has a visible 'swirl'. A friend and I and the shooter first noticed this 'swirl' for the first time 47 years ago and that was before we had ever heard of it... I don't even remember the rifle/cartridge at that time... and we didn't see the swirl every time fired... but we saw it enough that we puzzled over it.

We concluded there was possibly some combustion gases trapped in the turbulence immediately behind the bullet that resulted in a 'puff' we could see for a millisecond when everything was right... is there any truth to this? What does cause this 'visible' swirl.
 
guntech said:
I don't believe that every round fired has a visible 'swirl'. A friend and I and the shooter first noticed this 'swirl' for the first time 47 years ago and that was before we had ever heard of it... I don't even remember the rifle/cartridge at that time... and we didn't see the swirl every time fired... but we saw it enough that we puzzled over it.

We concluded there was possibly some combustion gases trapped in the turbulence immediately behind the bullet that resulted in a 'puff' we could see for a millisecond when everything was right... is there any truth to this? What does cause this 'visible' swirl.
Click to expand...

The friction between bullet and air causes a disturbance. This is what we see thru our spotting scopes. High humidity and sunlight play a big part in being able to see it.
 
Doesn't take much. A changing condition, shooting position or not looking in the right spot can all make it hard to see. Spotting swirl from the side through another rifle scope is never the best way/place to see it. Best place is above and directly behind with a spotting scope. The scene in the matrix where neo is bending over backwards to avoid the bullets and the swirl can be seen passing all around him is the easiest way to explain it to people who haven't seen it. Simply put the bullet pushes air out of the way as it travels down range. That disturbance(much like the wake of a boat) is what you can detect if you know what you are looking for. As the bullet travels down range it flies in an arc. As it reaches the peak of the arc(the culminating point) and starts to fall the rest of the downward travel may be hidden/blocked by the swirl the bullet has already left. Therefore when the swirl ends the bullet may strike LOWER than where you saw it disappear. Seen many a blown called shot because the spotter was going off of where the swirl stopped when the bullet was actually hitting a lot lower. I've also found that shooting over snow makes the swirl pretty hard to see depending on where the snow is located in relation to the swirl. If the backdrop behind the swirl is snow, it's going to be tough.
 
The O.P. lists 14 twist on the gun that produces visible trace. Plug 180 grain bullet and 14 twist into the Berger twist calculator http://www.bergerbullets.com/twist-rate-calculator/ There is a note that says "the BC of the bullet may not be optimized." I think that's because it will yaw all the way to the target and disrupt the air making visible trace. The minimum twist recommended for a 180 grain bullet is 12.8". Berger lists an alternate 115 grain bullet for greater stability in 14 twist.

In my humble opinion I think the slow twist is not delivering the bullet point forward and its yawing causing air disruption and visible trace.

D.C.
 
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