Feel the Heat

Is heat-re­lated POI shift al­ways down to your scope? Jim knows the an­swer

Airgun World - - Contents -

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.


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 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.

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