When shooting Field Target matches, being able to range with the scope’s parallax is critical to hitting the kill zone. Riflescope temperature shift can cause big problems!
Riflescope temperature shift can affect many scopes at certain temperatures. This will cause the scope to range shorter or longer than the true distance.
For example, under normal conditions, a scope will show the yardage as 30 yards. You can click or use the correct mil dot hold to hit the target.
When the scope shifts for temperature, the scope may range at 28 yards. That two-yard difference will have you miss the kill zone!
Keep in mind, not all scopes will have a shift with temperature. Some relatively inexpensive scopes won’t shift and then you’ll find a $1200 one that does. It depends on the scope…
Here’s what happened to me:
Our club had a late winter match in 28 F weather.
I had sighted in my Styer LG110 at around 80 degrees F in early summer. So that sight-in temperature will determine where you get a temp shift in your scope. I used a Sightron SIII 10-50 x 60 on this rifle.
I had no issues all GP season. As a precaution, I do cover the scope and rifle with a car reflective sun visor, cut to fit. This reduces riflescope temperature shift.
Also, I cover the gun right after I sight in at a match. This, because when you get it right on the sight in line, I want that accuracy throughout the match. So, covering the gun will help keep the scope and air tank around the same temp. Especially with a temperature-sensitive scope on the rifle.
Well, the temperature started at 28 degrees F and rose to only 33 degrees by the end of the match! That’s between 47 and 52 degrees less than the scope’s sight-in temperature.
Needless to say, when I shot at the first targets, I was way low. By the time I shot 4 lanes, I figured I needed 10 clicks up, to hit at 50 – 55 Yards. Using that method, range, click to the normal yardage and then add 10 more. That worked for a while. Especially with the sun on the upper course.
Then I went down to the lower section of the course. It definitely felt colder there in the shade and lower area. I hit a few, then did not drop another target for 8 lanes!!! I gave up and just scored for my partner the rest of the match.
On leaving the course I shot over the chronograph to see if the cold lowered the pressure in the air tank. It was 33 degrees at that time.
Nope! The speed was perfect. It would just shoot way low.
Let’s Define The Issue:
Now I am not a physicist or an engineer, but using basic physics and working with metal each day in my metal target business, I figured this…
In the scope, there is an erector tube you actually view through with your eye, It is made of aluminum. On the elevation and windage knobs, there is a stainless-steel bolt that reaches to the erector tube.
Between the tube and this bolt is a brass disc that looks like a flat thumb tack. The point of the tack shape fits into a small hole in the end of the stainless steel bolt. This keeps the brass from binding and twisting on the tube. Instead it swivels around in the socket of the stainless knob.
So that’s three different metals – brass, stainless steel and aluminium – all in contact with each other!
High school physics classes teach that all metals expand or contract with temperature. Also, each of these different metals react at DIFFERENT TEMPERATURE RANGES to the other…
I believe that this is the cause of parallax changes caused by riflescope temperature shift. My guess is that scopes that don’t shift may use the same metals to mate in their design.
How To compensate For Riflescope Temperature Shift:
After that disastrous match, I went home and figured out my temperature shift charts.
How I did it, was to go out on my 100 Yard home range, on the concrete bench and shoot at each Yard from 10 to 55 Yards in 5-Yard increments.
I did this for days and weeks, until I had shot from 28 degrees F to 70 degrees F (we get weird hot days during winter here near the NC coast).
I found out that at exactly 62 degrees F, my Sightron would go back to the 80 degree zero like a light switch! Seriously….
So, I made my several charts from 28 to 30 degrees F and two other temperature ranges.
If you look at this photo of the 28 – 30 degree F chart, you see three columns. The first column shows the actual distance from the bench.
I have asphalt core samples in the ground at each 5 Yard distance. I also have a marked tape measure and small targets, that I use when sighting at each yardage. Therefore, the distances are right on the mark.
In Column 2 you see what the scope is telling me is the yardage.
KEEP IN MIND THAT IT IS THE ACTUAL RIFLESCOPE TEMPERATURE SHIFT IN ACTION!
The third column is the clicks I need to add to the normal yardage, to hit the target. These are clicks up.
So, I ranged to the 10-Yard target. When clear in the scope or ranged, it read 9 3/4 Yards approx. on the side wheel. So, I put that in column 2.
Then I clicked to the normal clicks for 10 Yds. It hit about dead on. I put zero in column 3. Which means I don’t need to add any more clicks.
15 Yards came to read 14 on the side wheel. When I shot the paper target it was low. I held on the target dot and clicked up the reticle to the dot, counting the clicks.
It came to 5 additional clicks to get it to hit dead on. You get the idea…
Now if I ranged in that temp slot of 28 to 30 degrees, and it showed 28 Yards on my side wheel, I would look at the chart in the photo above. I can determine that the target was actually at 30 Yards, and I needed to click to 30 Yards and add 12 more clicks to hit it dead on.
Simple really!
You just need to figure where the scope comes back to normal and be aware of it all the time.
Below, Paul’s advice can be checked on the Field Target sight-in range.
I use a laser thermometer to check the temp of the HPA air tank. I also check the scope at its center where the knobs are. I carry it during the cold months in my gun cradle.
As the temperature climbs closer to that 62-degree switch to normal, I check it every lane. Once, it did bite me for two shots I missed, when I forgot to check it. After the two misses I did check, and it was at 62 degrees on the nose! Weird how that works…
Riflescope temperature shift occurs in high temperatures too. My scope would shift 4 Yards longer in ranging, once the scope and air tank was over 110 degrees F.
I avoid this by using the car reflector, cut to fit the gun. After sighting in at a match, I keep it covered to maintain near the temperature I sighted-in at. Works for me.
Now not all scopes suffer from this riflescope temperature shift phenomenon. I have a March 8-8-X60 on my Thomas rifle, and have never seen a shift yet. I was told that is does shift over 100 degrees. I have not hit that temperature range yet to see.
Why some scopes shift and others don’t, is work for that Engineer or Physicist, to determine. Too much for this old country boy’s mind to grasp…
Have fun and shoot well. Just watch out for riflescope temperature shift!
Paul Porch is the owner of Paul’s Custom Metal Targets. He makes beautiful custom field targets. Check out his web site here. He certainly shot extremely well at the latest CAAFTC, following his own advice in this post!
The post Compensating For Riflescope Temperature Shift In Field Target Competition appeared first on Hard Air Magazine.