WHY doesn't rain cause point of impact to change? - Page 2

Bart B. · July 24, 2014, 11:56 AM

That "halo" wogpotter refers to is called the "trace" in high power rifle competiton. It's best seen and used in rapid fire matches at 200 and 300 yards when a 20X to 25X spotting scope's placed directly behind and above the shooter in sitting or prone positions.

Each shot's trace was seen going into the target and in conditions when there were enough heat waves (mirage) to blur out bullet holes in the black aiming bull, that trace point into the target showed where the bullet went. After the first two shots were fired and the shooter was reloading, the shooter would call those two shots then the coach would give a sight change or, if both were centered, say "no change; shoot away" or words to that effect. The coach would tell the shooter when his shots went out from center so the shooter could hold off to compensate.

A good example I found online is this one:

https://www.youtube.com/watch?v=2SvrYKahCEc

Look carefully as you can see it coming up from bottom-center in the image then going above the target and starting to arc back down. In real life, the trace is much, much easier to see than that video shows.

Its use was also in the US Army Marksmanship Team's manual on sighting in a 30 caliber magnum at 1000 yards after bore sighting. The rifleman would lay down and shoot a round and the coach watched through a 24X spotting scope. As the trace reached its high point, the coach would give sight adjustments so the trace would appear centered left to right and above the target 2.5 times its size above it as it appeared. That's where those rifles put the trace when zeroed for 1000 yards.

wogpotter · July 24, 2014, 03:31 PM

I was actually thinking more of the white circular effect that you sometimes see in very high humidity, somewhat similar to the effect seen on aircraft wings. The trace you're thinking of is the pressure distortion. I'm more looking at the one caused by moisture re-condensation in the low pressure area directly behind the projectile.

boltomatic · July 24, 2014, 03:43 PM

I would think that the momentum of the bullet is just so much higher than that of a raindrop that there would be no noticeable POI shift.

Its like jumping up and down technically changes the course of the earth's orbit around the sun, but the change is so small that for all practical purposes there is no change.

With bullets, the change is small enough that the inherent inaccuracy of the rifle would mask any effect of rain.

July 24, 2014, 03:31 PM	#27
wogpotter Senior Member Join Date: September 27, 2004 Posts: 4,811	I was actually thinking more of the white circular effect that you sometimes see in very high humidity, somewhat similar to the effect seen on aircraft wings. The trace you're thinking of is the pressure distortion. I'm more looking at the one caused by moisture re-condensation in the low pressure area directly behind the projectile. __________________ Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”? Elderly Hunter: “That's dashed unsporting. Probably Belgium.”

July 24, 2014, 11:56 AM	#26
Bart B. Senior Member Join Date: February 15, 2009 Posts: 8,927	That "halo" wogpotter refers to is called the "trace" in high power rifle competiton. It's best seen and used in rapid fire matches at 200 and 300 yards when a 20X to 25X spotting scope's placed directly behind and above the shooter in sitting or prone positions. Each shot's trace was seen going into the target and in conditions when there were enough heat waves (mirage) to blur out bullet holes in the black aiming bull, that trace point into the target showed where the bullet went. After the first two shots were fired and the shooter was reloading, the shooter would call those two shots then the coach would give a sight change or, if both were centered, say "no change; shoot away" or words to that effect. The coach would tell the shooter when his shots went out from center so the shooter could hold off to compensate. A good example I found online is this one: https://www.youtube.com/watch?v=2SvrYKahCEc Look carefully as you can see it coming up from bottom-center in the image then going above the target and starting to arc back down. In real life, the trace is much, much easier to see than that video shows. Its use was also in the US Army Marksmanship Team's manual on sighting in a 30 caliber magnum at 1000 yards after bore sighting. The rifleman would lay down and shoot a round and the coach watched through a 24X spotting scope. As the trace reached its high point, the coach would give sight adjustments so the trace would appear centered left to right and above the target 2.5 times its size above it as it appeared. That's where those rifles put the trace when zeroed for 1000 yards.

July 24, 2014, 03:43 PM	#28
boltomatic Member Join Date: March 25, 2014 Posts: 98	I would think that the momentum of the bullet is just so much higher than that of a raindrop that there would be no noticeable POI shift. Its like jumping up and down technically changes the course of the earth's orbit around the sun, but the change is so small that for all practical purposes there is no change. With bullets, the change is small enough that the inherent inaccuracy of the rifle would mask any effect of rain.