45x scopes & mirage.

[BJS6]

Fred,

Could the small change in point of impact and the elongation of the 600 yard group in your tests be attributed to load performance/velocity changes, velocity created barrel vibration changes and/or flight characteristics due to changing atmospheric conditions over the day.

Those are some very small groups at the ranges you were shooting, what calibre etc ??

Bryce

 

[Fred Bohl]

Bryce,

The setup group was round. The same bullets and loads lots were used. That lot of loads has previously showed max ES of 22fps and mean of 2920fps. The only significant change was atmospheric from 54deg 44%rh and no visible mirage to 78deg 60%rh with strong “boil” mirage visible. We therefore believe that the group center position change and tendency to vertical dispersion is primarily due to the lower air density (warmer and wetter) and more air turbulence (as evidenced by the vigorous “boil” observed) altering the in flight drag as compared to the setup group and even shot to shot within the 1 minute pause between shots within the group. We also still believe that the longer air path and time of flight at 600 yards is needed to make the effect large enough to be distinguishable.

Thank you for the complement, but the actual shooter was my associate who prefers to remain anonymous (but did admit to selecting the best example groups). As to equipment used:

6Br – Norma Brass (4th reload, neck sized only), CCIBR4 primers, Reloader 15 powder (30.8gr), Berger 105gr VLD (0.010 jam) selected bullets.

Rail Rest – borrowed Magnum Metal unit - used action mounted Weaver T-36 FCH scope on Ivey BR30-050 with short action cam [0 to 50 MOA on 5.5 inch rail] mounts and 30 mm to 1.000 inch adaptor bushings.

My #2 Barreled Action - Stiller Cobra action [right bolt, left port, no ejector] fitted by Stiller with Lilja barrel [1.45 diameter 3 lands and groves 6mm (0.243 grove, 0.237 land diameters) 1 in 8 twist] 26 inch finished length chambered for 6BR [0.272 neck diameter, 0.104 free bore] using custom PT&G reamer. Jewell BR trigger [2oz.].

During other testing in a railroad tunnel (no wind or mirage), the above configuration has delivered 20 groups with that and similar loads in the 0.08 to 0.09 MOA size range. And the same barreled action and loads have delivered 20 groups with that and similar loads in the 0.09 to 0.10 MOA size range in my Bill Shehane Maxi Tracker stock (aluminum body - split barrel block) on Shehane front and rear adjustable rests.

 

[Tim Oltersdorf]

Shimmer little light beam

I have shot 36, 40 and 45 power leupolds. Mirage is worse the higher the power of the scope. Mirage is not something to be compensated for but a useful tool to improve your shooting. Try shooting just the mirage and see what happens. Try combining it with the wind flags. It can tell you a lot. Tim

 

[Fred Bohl]

Tim,

I have to admit that more than any other factor, my inability to "read" the wind drove me out of BR competition. Wind flags have been a source of as much confusion as a help.

However, since I've been experimenting with the rail gun, the mirage problem and how to deal with it, things are starting to improve. I'm doing much better in a combination of wind and mirage by "reading" the mirage as an indicator of wind and then using that info to compensate for wind.

For me at least, more scope power (magnification) is better for both finding the real target location and for interpreting the wind effect from the mirage appearance.

 

[Boyd Allen]

The sun shines on the ground and heats it. Air that is in contact with the ground is heated by it to a greater extent than that above it. The warmer air is less dense. No moisture is required for this to be true. The lighter air rises, and as it does is pushed around by any wind that is blowing. because of its different density it refracts light differently than the colder air that is is rising through. This causes the target to appear to be in a slightly different location that jumps around because the hot air rises through the colder air in an uneven manner, sort of like the bubbles in boileing water. If one aims at the higher location that the target is jumping to as well as the lower position that it snaps back to, while shooting a group. it will be enlarged because the target is not really moving and you have aimed at two different spots. I believe that learning where to aim when there is mirage is mostly a "cut and try" proposition. Waiting for it to go away to aim would only work if one was shooting free recoil, where the gun is not touched while or after being aimed (except for pulling the trigger). On many days the mirage does not stop.