Feel the Heat
Is heat-related POI shift always down to your scope? Jim knows the answer
Sunday 25th June was blisteringly hot, and also the day of the second round of the excellent UKAHFT 2017 series at Buxted. A friend who attended the event told me that the temperature hit 33C in the afternoon, and that quite a few people had reported that their pellet’s point of impact (POI) had shifted due to the heat. It’s actually common to get pellet POI shifts with temperature changes, but the reason is usually the rifle.
With spring airguns, POI shifts can be an inch or more, and are mainly due to the piston seal swelling when hot, and shrinking when cold, altering the seal’s friction and the point in the recoil cycle when the pellet exits the muzzle, so it is pointing higher or lower than usual at pellet exit. With PCPs, POI changes are much smaller, and caused by the molecules of the air in the cylinder possessing higher kinetic energy, which raises muzzle velocity, (or vice versa) as discussed last month. The suspicion at the Buxted shoot was falling not on the rifle, but the scope. In theory, a scope could alter the POI with changes in temperature and to understand how, we need to take a look inside the scope.
INSIDE THE SCOPE
The mechanism that alters pellet POI when we zero comprises a tube containing lens elements and is called the ‘erector’ tube, which is free to pivot at the rear, and held at the front by a triangular arrangement comprising the elevation and windage adjusters, and a spring. To move the POI left, the windage adjuster is usually screwed in toward the axis of the scope, which pushes the front of the erector
“The suspicion at the Buxted shoot was falling not on the rifle, but the scope”
tube left, and to move the POI down, the top adjuster is screwed in, pushing the erector tube down toward the axis of the scope, and vice versa.
So how could temperature move the erector tube to change the POI? It’s all down to materials swelling in heat and shrinking in cold, which means that the length of the adjusters increases or decreases, moving the erector tube. That is complicated by the fact that the adjusters are held in the turret blocks, themselves secured on the scope body tube, and the expansion or contraction of these acts in the opposite direction to the adjusters. If an adjuster increases in length because it’s hot, the body tube also expands, pulling the adjuster back with it, and potentially cancelling out, or mitigating any effect on the erector tube.
The only way to address a matter like this is to get measuring the effect of heat on the individual components, and as luck would have it, I had kept an old scope that I’d opened up some years ago to photograph the internals.
The reported POI shift ranged between 5mm and 25mm at 30 yards, so I first needed to know how many clicks of the adjusters those figures equated to. A click is ¼” at 100 yards, and that’s 1.9mm at 30 yards, so a 5mm to 25mm POI shift would equate to 2.5 to 13.15 clicks of the adjuster, respectively.
Next, I needed to know how much the adjuster moved the erector tube per click, so I removed the adjuster assembly from the scope and measured it at 0.009mm. So, a 5mm POI shift at 30 yards would need the adjuster to increase in length by 0.01mm, a 25mm POI shift by 0.05mm. The question was whether or not the adjusters could expand quite that much.
In an ambient temperature of 27C, I measured the length of the (brass) adjuster at 12.985mm, then popped it into a deep freeze to chill to -7C, giving a temperature range of 34C, which I felt should surely be enough to replicate the range between typical British summer and winter averages.
After an hour, I removed it and re-measured; the adjuster was now 12.977mm, so the adjuster had shortened by 0.008mm, just under one click of adjustment and equal to a POI shift of 1.69mm at 30 yards. To check my
measurements, I then needed to do some calculations based on widely published coefficient of linear expansion figures.
The coefficient of linear expansion is given as a number; for brass it would be 19, multiplied by 10 raised to the power of minus six, which gives the expansion per millimetre per degree C, so the adjuster should have expanded by 0.00839mm. practically one click of adjustment, which would shift the 30 yard POI by 2mm or thereabouts. The slight discrepancy between my measured and calculated figures could be due to the ‘brass’ adjuster actually being an alloy, or perhaps one of my temperature measurements was out. but in both cases, the potential POI shift attributable to the scope is a fraction of the shift reported at Buxted.
Whilst the adjuster is expanding toward the axis of the scope as it gains temperature, it is being pulled back out by the expansion of the tube on which it is mounted. The scope tube would be difficult to measure precisely because it is almost certainly not perfectly round, so I decided to stick with calculation of linear expansion. Aluminium has a coefficient of thermal expansion of 22 X 10 to the minus 6 mm per mm per degree, so the 25.4mm body tube diameter between -7 and 27C would increase by 0.018mm. This would pull the adjuster back by half that amount, which is 0.0095mm.
The net movement of the erector tube of the scope body and adjuster expanding would be in the order of 0.0011mm, about one eighth of a click of the adjuster. If there is POI shift, it would be far too little to measure at the target.
In order to move the POI, you have to alter the course of the light travelling through the scope relative to the reticle, which is what the erector tube does. If we discount the erector tube mechanism, the only other possibility would be for a lens element to tilt away from the axis of the scope, which I don’t believe is possible.
HEAT AND US
Heat affects not only your rifle – and, to a tiny degree, your scope – but it can also have a huge effect on us. The Buxted shoot came during a period of five consecutive days with temperatures at or above 30C, and for those five days, I struggled to complete Sudoko puzzles that I would normally finish in very short order, which I think was due to difficulty in concentrating as a direct result of the heat. In HFT, concentration on the range, wind and the target is crucial; if your concentration isn’t 100%, your scores will plummet.
Physiologically, heat is draining, sapping your energy, so everything becomes more of an effort, even moving from peg to peg on an HFT course, or getting up off the mat after a shot, which does nothing to help your marksmanship, and of course, the heat makes us sweat, which is a problem when trying to replicate spring airgun hold with clammy hands. On the day of the Buxted shoot, some who stayed back to shoot at Nomads were complaining about sweat running down into their eyes, which is hardly conducive to accurate shooting.
If the scope used for my measurements was representative of other scopes, and there is no reason to believe otherwise, then a shift of 34C in temperature could potentially move the POI by about a quarter of a millimetre (one hundredth of an inch) at 30 yards, so not only were the scopes at the Buxted shoot innocent, but it is looking fairly unlikely that any significant heat related POI shift is ever due to the scope.
The adjuster housing screws into the turret block, through a neoprene washer to keep water out.
I’ve never suffered temperature related POI shift with my PCP.
If a scope does not respond to small adjustments in windage or elevation, the erector tube is probably dragging on the leaf spring, visible here.
The erector tube is free to pivot from the rear as the adjusters move the front.
One click with most scopes equates to less than half a pellet width at 30 yards.
The adjuster and its housing.
With the adjuster in its housing, I was able to measure easily how much one click moved the erector tube.
I measured the length of the adjuster at a range of temperatures from -7c to 27C.