Mirage and conditions that effect target image in rifle shooting

[Gunlaker]

Realistically if I try this one day I'll just use a sturdy scope stand and make an adapter for my big Unertl scope. Sort of like a transit.

Chris.

[BrentD]

Just do it in your garage, maybe looking down a long, hot street paved in asphalt. I can't believe there will be much movement at all.

[M-Tecs]

Fiberglass thermal expansion is about 0.00014" per inch per degree. If the transit has 50" legs a 10 temperature change will be .070". If all three legs change the same amount it not an issue since the crosshairs would only move .070" parallel to the target, however if one leg is warmer or colder it becomes an issue since the movement is now angular and not parallel. Even a one degree temperature difference on one leg will show substantial angular movement.

0.00014" X 50" = .007" for a one degree change. If the temp change is 10 and the transit leg is 50" long you get a .070" change.

http://www.inspectapedia.com/exterio..._Expansion.php

[Gunlaker]

Interesting. So it's quite likely that any effects people see are to be attributed to this sort of thing rather than th changing refractive index of the air.

Chris.

[M-Tecs]

Interesting. So it's quite likely that any effects people see are to be attributed to this sort of thing rather than th changing refractive index of the air.

Chris.

Anytime one observers' an effect that doesn't have a scientific basis false attributions are mostly likely the cause. Every aspect of external ballistics are scientifically understood and documented by the corresponding scientific formulas.

My question is were are the formulas that deal with a shifting point of impact for scoped rifle shooters related to the position of the sun???

Were is the discussion from the long range competitors, professional snipers, pre-electronics artillery men and pre-electronics surveyors related to the position of the sun ???

The closest you will find is Snell's Law. As a bow fisherman I deal with this all the time, however, since air is only one medium it doesn't apply to this discussion. Snell's Law deals with the optical effect of refraction as light passes from one transparent medium to another. Light changes speeds and bends in different mediums. How much this happens depends on the refractive index of the mediums and the angle between the light ray and the line perpendicular to the surface separating the two mediums. Each medium has a different refractive index. The angle between the light ray and the normal as it leaves a medium is called the angle of incidence. The angle between the light ray and the normal as it enters a medium is called the angle of refraction.

Snell's Law
In 1621, a Dutch physicist named Willebrord Snell (1591-1626), derived the relationship between the different angles of light as it passes from one transperent medium to another. When light passes from one transparent medium to another, it bends according to Snell's law which states:
Ni * Sin(Ai) = Nr * Sin(Ar),

where:
Ni is the refractive index of the medium the light is leaving,
Ai is the incident angle between the light ray and the normal to the meduim to medium interface,
Nr is the refractive index of the medium the light is entering,
Ar is the refractive angle between the light ray and the normal to the meduim to medium interface.

[M-Tecs]

Back to the OP's subject. Another good link on mirage http://blog.cheaperthandirt.com/read...nd-and-mirage/

[EDG]

If you have questions just talk to a surveyor.
A coworker was a RPLS (registered professional land surveyor). He talked about taking a lot of time letting the system stabilize before recording the numbers. He said it was very important when surveying large tracts of commercial zoned property that was worth millions of dollars.

[44man]

I did not have the scope on a gun and was in shade all day. Very strong bench too.
If you looked through long enough you could watch the cross hairs creep. Very calm day so I did not see much mirage.
What was interesting was the cross hairs followed the sun, sun left hairs left, sun up hairs up.

[Sharpsman]

Reading wind and mirage is the fun part of long range shooting. One of the best ways to learn both is to take up NRA High Power and shoot as many of the team matches as possible. For the team matches you are allowed to coach. Coaching a Master or High Master shooter gives you instant feedback on your wind and mirage reading abilities.

What kind of table was the scope mounted atop; wood, metal, concrete, etc.????

[M-Tecs]

What kind of table was the scope mounted atop; wood, metal, concrete, etc.????

http://competitions.nra.org/how-to-g...mpetition.aspx


http://www.thetruthaboutguns.com/201...ra-high-power/

[Don McDowell]

M-Tecs have you ever shot a blackpowder target rifle match?

[M-Tecs]

Mostly shoot NRA High Power Palma and F class but I do some BPCR out to a 1,000 yards.

[Don McDowell]

Intersting. Then I'm sure you've become well aware of the intense awareness of the wind, mirage and light that bpcr target rifle takes. Most Highpower shooters don't convert well to bptr, as the stuff that won't even make a high-power flinch even in the midrange stuff can and will cause a miss with the bptr loads. I would imagine anyone that is even moderately successful with bptr, would find high-power somewhat boring in it's simplicity.

[BRUCE MOULDS]

don,
we all shoot in the same wind and same mirage.
these things can be your friend in competition if you know how to use them better than the competition.
I have found fclass and bptr to be great cross training for each other, and would rate shooting 100 in bptr to have about the same degree of difficulty at 1000 yds as 100.10x in fclass.
have never achieved either personally and probably never will.
sling shooters suffer buffeting that cross stick shooters do not have to wear as much.
light affects iron sights whatever the discipline.
the vert that comes with mirage affects any sights, scope included.
bptr bullets do seem to get more vert from crosswinds and be more affected by windshear than vld smokeless bullets.
the windshear can be minimized by faster twists, but this can be offset by more vert effects in crosswinds.
bptr scores will mostly be lower than smokeless due to level of difficulty in making correct wind corrections.
however good bptr shooters can beat not so good smokeless sling shooters in off rifle scores.
usually vert induced by mirage is less than 1 moa, so if you have your elevation settings pretty right, a bit of boil, while making your shot go high, will still be in the 10 ring.
you just have to shoot in the wind and keep learning until you die.
I will confess that shooting in winds that change direction fast and vary in strength from25 to 40 mph, is extremely hard with bpcr.
one thing that makes bptr a bit harder is that when wiping, or even blowtubing , there is a lag between shots which makes the next shot more of a sighter than when shooting smokeless.
you cannot "chase shots" as much.
chasing shots has never been a good policy anyway. knowing what is on the sight and relating that to the condition is the way to go.
what bptr needs to get over is having spotter/wind coaches.
the shooter alone should do this job for themselves in a pure sport.
keep safe,
bruce.

[Gunlaker]

A lot of people look at mirage as a problem, but it's really your friend as it can show you things you can't see otherwise.

I agree that it would be nice if shooters were required to do their own wind calls.

Chris.

[Don McDowell]

Bruce, it's flight time, the flight time of a 45 bptr bullet to the 1000 yd line is a bit over 3 seconds. A lot can happen in that short time.
You need to come shoot the Rocky Mtn regional at Byers, or the Nationals at Raton sometime. I don't ever remember a headwind dropping a high-power round over 2 minutes the way a switch from cross to head can do with the bptr.

[country gent]

Mirage will show wind changes quicker than a flag does if you know how to read it. Whats interesting is to get out on the range a little early and watch the wind mirage changes, sometimes there is a pattern to them. A treeline or other barriers along the range and flags may "lie" to you. Viale range at perry points 1-20 or so are a tunnel due to the fence row along the side of the range. I have shot 2 clicks right at 600 yds for a clean score with the M1A and a buddy down on point 100 or so had 10 on and was left and right 2-4 clicks thru his string. Mirage is read at your target not one end or the other of the range where it might not matter as much. Learning to use all the indicators and trust your judgement is a big plus. I perfer to shoot in the same run or wave pattern of mirage a true boil and my eyes dont work. A shooters notebook / scorebook can also be your friend.

[BRUCE MOULDS]

don and country gent,
you both make good points.
during 3 seconds time of flight there can easily be a wind change.
this is beyond your control as the bullet has left the barrel.
actually you need to add to that 3 seconds time taken to get from the scope to the gun, check the level, take a good aim, and also to check that you are not going to crossfire.
this along with greater difficulty getting super accurate ammo, non free floating barrels, more primitive sights, etc, etc etc, means that we have a greater degree of difficulty than a palma shooter with a 308.
we also have to use more energy in a string wiping or blowtubing, and we tend to be old men.
I have noticed that when 308 sling shooters start getting possibles, I shoot about 10 points off a possible, but they are probably better shots than I am.
should we consider what fclass did, and use a target that reflects our degree of difficulty.
fclass went 1 ring smaller and halved the x ring, and this worked well for them.
if we made the 9 ring our 10, and kept our current x diameter, we might have a target that reflected our degree of difficulty.
the odg kept very good notes. some of the old books available on the internet free give good examples of this.
they included barometer readings, wind calls, sight settings, temperature mirage, wind strength and direction, and many other things.
you need one of these for every range.
most of us do not consider the barometer theses days.
I recall an English shooter at creedmoor commenting on the barometer dropping a huge amount during a string.
we are still learning the bptr craft as a group, so we do not know what final expectations can be.
what we do know it is a hard discipline. perhaps this is the attraction.
keep safe,
bruce.

[Don McDowell]

Bruce I find the challenge to be the best attraction.
I know a fella that shoots High Master in Highpower. Three years he attended the nationals and some regional matches. He was unable to break out of Sharpshooter. He quit. He does still shoot gongs, but he says the target rifle is to hard, and to many things that go wrong, it's to frustrating and he's just not going to do it any longer.

[Chill Wills]

Very good post Bruce.