(From the undiscovered, secret “Art of the Rifle” archive vault of shooting past. I thought I would take a break from the number crunching of scope testing and write about something quick and easy. Apparently I have a mental disorder that compels me to collect and crunch numbers.)
Parallax seems to be in the mind of more shooters as they are starting to notice that a lot of scopes have side focus knobs. There’s a knob there that says “Parallax” with yard numbers and such, so it has to be a big deal. I must have noticed that knob at some point, as evidenced by this old article on my blog.
Before I entered the abnormal mental state of coming up with numbers to crunch in relation to shot group stats, I had a different abnormal mental state of coming up with creative shooter-related diagnoses for what turned out to be a gun that needed some accuracy work. One of the things I got hung up on was parallax, as my scope on that rifle, formerly the FN PBR-XP, currently the “Mark Deux”, is not equipped with a side focus knob. Sometimes not having the means to adjust something can give one the idea that that lack of that ability is a serious and significant deficiency.
Earlier in the year I was loaned a sample of the upgraded SWFA 3-15×42. Perhaps upgraded is the wrong word, because I don’t know that it’s actually intended to replace the 3-9×42, but it would just make sense. As Nigel might say, “This one goes to fifteen. Well that’s six more, innit?” I thought it would make sense to compare them and see how that turned out.
The newer 3-15×42 has a side focus knob. One of the things I wanted to compare was whether that resulted in better capability for precision. Before I did that, I needed to establish a baseline with my tried and true 3-9×42. What follows is my submission of evidence that I actually made my journey to the dark land of parallax and returned to tell the tale…
Plan A: “No plan survives first contact with the enemy.”
Armed with the On Target TDS program and some of the proprietary targets that go with it, I set out to attempt to induce parallax error at 100 yards. Those of you who are smarter than me (only the 7,000,000,000 who have that distinction) may have already spotted the flaw.
I have to reverse engineer my grand plans to some degree, because this happened a couple months ago, but the plan seems to have been to shoot 3-4 sessions of 12 shot experiments. Each target consisted of three columns of four bulls. For those actually keeping track that’s one shot per bull. Since I had three columns, for the left column I would put my eye as far left in the scope’s eyebox as I could while still being able to see the target. For the center column I carefully aligned my eye in the center. For the right column I moved my eye to the far right. I shot them from left to right, which would mean that each group was formed in a “round robin” fashion to reduce the possibility that something would make one group better than the others due to some outside condition. After a few outings I should have collected large enough sample sizes to be able to tell something.
After shooting the 12 shots I compiled the results into three groups of four shots, one for each column, using On Target TDS. On target allows for precise measurement of group size in extreme spread and mean radius, the location of the exact group center, and the measurement of the distance of the actual group center from the point of aim in terms of horizontal, vertical, and total deviation.
The raw targets look like this:
Here are the composite groups:
After analyzing my first target I remember being confused. In retrospect I should not have been thinking anything after only four shots on target, but I did not see what I had expected to see, which was three distinct points of impact that correlated directly to the directional change in eye position. Nothing in life ever turns out that perfect.
Later in the day I tried again with exactly the same format. This time I felt like I shot a little better. Note that in this case the group centers were all very similar. There are differences in group size, but I felt that could be easily attributable to having more difficulty with sight picture and eyestrain at the edge of the eyebox.
You might have noticed little to no shift in the point of impact in the last three shot groups and the composite total. I noticed that too. I now had a total of 24 rounds telling me something different than I expected. I started considering something that I should have thought of before shooting. I emailed SWFA and asked them what the parallax was set to in the scope. Skylar answered the same day with the following: “The SS 3-9 has a fixed parallax set at 100 yards. “ Trying to induce parallax error at the precise distance that the scope is set to be parallax free is not a really good idea for those of you that are sane and well-adjusted.
Plan B: Trying to induce parallax at a distance other than the factory setting.
After hearing from Skylar, I set about researching the problem a little more. I found something interesting on the internet here: http://www.longrangehunting.com/forums/f18/parallax-adjustment-scope-57372/ (scroll down to post #6 by LouBoyd).
Using his equation, RT = RS(Dt-Ds)/Ds, I figured out that at 200 yards the possible parallax error was equal to the radius of my objective lens, or approximately 0.827”. I used 200 yards because of my overdue shooting goal, which is to be able to hit a 4” target within that distance under a wide variety of conditions (understated). 0.827” isn’t much, but I decided to test at 200 anyway.
I placed three targets at 200 and shot them in round robin fashion, with 6 shots each. Here you go:
Here is a composite target of all 18 shots:
Here is a chart of the deviation, in MOA, of each group.
Here is a chart of the mean radius if each group, also expressed in MOA:
After seeing that the group that should have been pretty good was not, and that the groups that should have not looked as good, but did not, and after not seeing any particular shift in point of impact, and, after seeing that my groups at 200 yards in which I tried to induce parallax were better than the groups I fired at the distance at which my scope is parallax free… I decided the following:
While parallax is a real thing, the importance of it isn’t universal to all shooters. For long range it’s going to be more important. For my purposes, there are more important things to work on, like follow through, which is what I think happened with the “center eye” target at 200. Once again, it comes down to fundamentals.