BoydAllen
Gold $$ Contributor
If one were primarily doing pressure and velocity evaluations, or using a return to battery setup to test accuracy, tunnel mirage would not be much of an issue, but for evaluating accuracy to the highest levels, using rifles that must be reaimed for each shot, I am told that it is. The value in an expansion room, or chamber, is that combustion gasses are not projected into the tunnel itself nearly as much as they would be if no chamber and baffles were incorporated in the design. This makes it much easier to keep the tunnel air in a more uniform condition, minimizing optical distortions. Another issue is the difference in temperature of tunnel walls and the air within the tunnel. This is more of an issue for tunnels that are above ground. Some tunnels are better than others, and once they are finished, it may not be practical to remedy their flaws. This is why I think that anyone planning one should do a lot of research before finalizing a design, or breaking ground. BTW, I don't think that Gene Beggs would disagree with any of what I have posted here, except perhaps the manner that I tried to make my point in the first one, which, upon reflection, I disagree with myself.