Impossibility (?) of eliminating temperature-induced P.O.I. change in a breakbarrel springer.

Thoughts on temperature-induced P.O.I. (point of impact) changes:

All my airguns are springers.

Of my springers, the following do change P.O.I. (hit lower as the day heats up):

.1. .22 HW80K

.2. .177 FWB124d

.3. .25 Webley Patriot/Kodiak

And the following do not change P.O.I (hit lower as the day heats up), at least that I have observed:

.1. .177 TX200 Mk3’s

.2. .177 TX200 Hunter Carbine

.3. .177 Beeman R7

.4. .177Beeman R9 (HW95)

.5. .177 HW55’s

.6. .177 Beeman R1 (HW80) (8.1 ounce piston made by Ed Canoles w/o-ring sealing replaced the factory 12 oz piston)

.17. .177 IZH60

All my airguns are tuned.  Some quite a bit.

But I have no solution to the problem of a springer that in the cool of the morning of a farm pest hunt is zeroed correctly and by midmorning of a day that is heating up, hits 1” low at around 45 yards and 2” low at about 45 yards by noon, even for a dawn to noon temperature range on only about 60’F to 75’F.

For example, my 13 ft/lb .22 HW80k shoots 13.43 gr soft lead, round head AirArms Falcon pellets about 705 fps near sea level.   It’s two zeros are 10 yds and 43 yds.   At the beginning of hunts, in the cool of the morning, I check sights by shooting a group at 43 yds and it is right on.  By midmorning it is hitting 1” low at 43 yards.  By lunchtime it is hitting 2” low at 43 yds.  

This .22 HW80k is tuned:

.1. 13 ft/lb Maccari tune kit (similar POI problem with Maccari and Vortek full power kits)

.2. 12 oz factory piston reduced in weight to 8.7 ounces (Ed Canoles)

.3. o-ring sealed piston (Ed Canoles)

.4. VERY thin coat of Beeman moly grease on spring and spring guide (VERY thin)

.5. VERY thin coat of grease on piston seal and compression cylinder wall.

.6. Absolutely NO Maccari tar, be it velocity or regular tar in the airgun.

.7. fairly lightweight Swift 40mm 4-12x A.O. scope with 1” tube

.8. BKL one piece mounts with scope stop.

.9. All seals not leaking and all screws snug, not moving.

It is very accurate, easy cocking and less noisy than a full power kit.   It is, for me, a “60 yard airgun” because on at 43 yards, 2” low at 50 and 4” low at 60 yds.   I am pretty good holding over up to about 4” but less so for more than a 4” holdover, especially in winds, so I try to limit shots to 60 and under with this airgun.

It is VERY sweet shooting but very annoying to have POI gradually drop as temperature gradually rises.   Having eliminated every other factor I can think of, I conclude the design of the HW80 is such that rising temperatures expand something in the breech area/detent lockup so the barrel slowly tilts downwards as air temperature slowly rises. 

I have given up on a fix, because I can’t think of anything else to try.  

This is not “hold sensitivity”.

This is P.O.I. shifting due to barrel angle changing with rising temperature.

Oddly enough, my .177 Beeman R9/HW95 does not ship zero with changes in temperature.   It’s build quality is less than the HW80’s build quality, but perhaps because it was a later design, the design is different in some way so that a warming day on a hunt/shoot does not see a dropping point of impact.

I believe it is in the lockup design.

RHamilton, California

btw, I am quite pleased with the new ($38 plus shipping)Vortek muzzle brake.  It is almost 5” long, extends the barrel about 2”, does not quiet but did seem to increase my accuracy in that in my farm pest hunt sight in, 5 pellets at 43 yds, sitting on a folding camp stool, resting on a tripod camera stand field rest, 3 of 5 pellets were touching and the other two, which was me pulling the shot off, were within about ½ inch. 

2017 has not been a good year for farm pest hunting here.   Four places closed off to me…(1) one because of ill health of the owners; (2) one because of urban development places; (3) one because a co-owner was very anti-gun/hunting and perhaps he, seeing the school etc shooting news, decided guns/hunting is evil, and (4) my best place due to the owner quitting the livestock business.  I still have access to three locations, but few or no pest birds, just California ground squirrels. …no concentrations of ground squirrels, rather thinly scattered so that I can hunt a half day, cover maybe 4 miles and get a dozen or so is all.   So the emphasis stops being high kill numbers and has become more of pure hunting and the goal being making good shots at longer ranges.   Which is fun also.   Long range skills.

That said, I rarely post and don’t do hunt write-ups because there is nothing much new to say.   Say I hunt a multi-thousand acre location.   The ground squirrels are only in certain areas.   A lot of acreage is pest critter-free.   There are occasional coyotes, but a .177 Beeman R9 is hardly appropriate.

R.Hamilton, March 2018 California

 lock up is  being affected,  detent latch spring   try to shim it  or replace  it.

is the pivot bolt tight enough and  whats going on with the breech seal is  it a newer harder aftermarket  or original softer  one.  if softer maybe a shim may be worth a try, I dont like the  harder new aftermarket breech seals

Hi Robert, the lock up is being affected. It could be the difference between a wedge type lock vs. a ball type lock. Also the grade of steel used in the barrel block, different grades of steel will respond differently to temp. changes.

It's probably too much clearence around the spring loaded ball or wedge. You might consider using a small pointed punch to put a small dimple on the barrel block above the wedge or ball to lessen the clearence a bit. hth

This is interesting. A properly functioning lockup mechanism, when closed, maintains a fairly high spring force against an angled surface to keep the barrel block pressed tight against the breech. It seems to me the beauty of this type of mechanism is that it does not require super precise manufacturing tolerances. So I wouldn’t be surprised to see a problem from time to time in a cheap model off the Walmart shelf but why would this be a problem in a Weihrauch and Feinwerkbau?

I've got a number of TX's. Mk II's & III's, and a Mk II tuned with a custom piston to match Mk I stoke. All are temperature sensitive. Almost 2 MOA at 55 yds from 60 to 90 F in one case. 

The expansion rate of steel is .0000063/inch/degree, as per a current edition of machinery handbook. It is not appreciably different for allow until the iron content gets less than 70%(highly alloyed moon rock materials). Cast iron has somewhat less

So if a 1" block of almost any steel used in a gun warmed 100degF, it would expand .00063"..I think it might be something other than the expansion of the metal.

Maybe it's the temp. and density of the air being compressed and the temp and density of the air the pellet is passing thru on the way to the target. Shooting in my shaddy back yard is different than the open field at our monthly fun shoot. I keep the guns in the shade under my pop-ups. I re-zero if I have to.

You didn't indicate if your AG's are scoped but if so, any chance that your scopes may be affected

by temp and not your guns? It might be worth looking into as I've seen references to this

issue in the past. Just a thought.

To really get a handle on it I suppose some chronograph and bc data would be in order. As temps change..Might be fun while you wait for the squirrels to breed. 

Just some observations from one guy's use.

All of them were a bit off at the first shots after a long lay off. The well tuned ones were both less off and got with the program faster (fewer "off" shots). Now that could be the human element getting use to shooting springers rather than the springers themself...but I tend to disbelive that.

Reason I disbelive it, is that I tend to shoot the springers in batches....so I'll shoot three or 4 of them at a sitting...and if it was just getting back into the "springer groove" shouldn't show up ass it does by the 3rd or 4th rifle.

So....what an change the accuracy/ perfomance?

Temperture.

Cold vs hot could change three mechanical things.

1. Seals limber up as they warm, and the compression or air provides the heat.
2. Lube softens/redisteubites as it warms up.
3. Very minor, but temp contracts/expands metal parts (including the scope).

So I played with those varaibles for a bit, pre-warming cold guns and pre-cooling warm guns.

And then a little bit of a "DUH moment".

In the cold I was wearing a padded jacket...in the heat I was in a T-shirt.

I still can't cure it...but if I shivver in the cold wearing a t-short and sweat in the warm wearing a jacket, it seems to take all of them fewer shots to get back to expected normal.

Re: DUH MOMENT

That's the same reason many a field target competitor's scores are higher when it's a little cool - cool enough to wear another layer, resulting in less wobble, pulse, etc.

I hate shooting ft when it's hot.

.1. I forgot....along with my AirArms TX200 Mk5, TX200 Mk3, TX200 Hunter Carbine not moving P.O.I. as the day progresses from a colder early morning to a hotter early afternoon, also my .22 RWS54 (factory piston, Ed Canoles o-ring sealed piston and full power Vortek tune kit (14.3 gr Crosman Premier domes at 812 fps....formerly 900 fps*)....my RWS54 is keeps it's P.O.I. despite rising temperatures.

.2. All my hunting air rifles are scoped, using one piece mounts (BKL mostly, but also BSquare and Beeman/Sportsmatch).   If it is (quite possibly) as case of metal exspansion in breakbarrel lockup areas, then using "all on the barrel" iron sights might be a solution for others...but not for me because my aging eyes can't use the factory open sights.  I can use a peep but peeps don't go on the barrel and so are not a solution if the barrel angle changes with temperature.  Plus iron sights don't make it for me on small pests at longer airgun distances....such as the head of say, a rat at 50 yards.   

.3. All my scopes are fairly light....most all 4x12x A.O. 1" tube scopes.  I have larger scopes on some of my non-sensitive airguns, which don't cause problems.

.4. My breech seals are o-rings (FWB124d) or Beeman breech seals, which squish down ok, or seem to.  Vortek has an interesting breech seal.  Anyone try it?

.5. I'm looking for someone who has solved the breakbarrel P.O.I. temperature sensitive issue, though your ideas may well be solutions....the idea of a more soft breech seal is interesting.

.6. I am totally floored that a TX200 could be temperature sensitive since none of my fixed barrels are.

.7. As an aside, I don't do long hunts anymore with my .22 RWS54 because at only a bit over 800 fps, it doesn't have the effective range it used to.  My first Vortek kit did 14.3 gr CP's at 900 fps.  Ground squirrels in the open at 80 yds were slam dunks and I was getting them at 90 yds.   But over a year or two, fps dropped to about 855 fps.   Still ok, but then I got a newer "more pop" Vortek spring and put it on the original guides and it only did about 815 fps.   I could not get it off so to go back to the original spring, so I bought another spring...same thing...about 812 fps with 14.3 gr CP's.    For the bulk and weight of my RWS54 to be worth lugging up hill and dale, I want 900 fps.   As is, my .177 Beeman R9/HW95 is smaller, lighter, just as accurate and doing 8.4 gr JSB Exact 4.52's at 925 (clean barrel) and is, for me, and 80 yd gun.

.8. Detent lockups vary....ball detent on my temperature sensitive .177 FWB124d.   But also a (larger) ball detent on my non-temp sensitive .177 Beeman R7/hw30s.  Chisel detents on the non-sensitive R9 but chisel detents on the temp sensitive HW80k.   The 80k went thru two different compression tubes (I like non-honed for o-ring sealed pistons).   The .177 R1 is not temperature sensitive that I have noticed but then again it is so tuned and modified, it's own Mom barely recognizes it....I will say that with my R1, the Maccari kit gives a tiny bit more power but there is some vibration.   The Vortek kit is totally vibrationless.  (I have a lot of kits)

R.Hamilton

Personally I enjoy the .22 HW80K more than the .177 Beeman R9....just for shooting feel, and would dearly love to solve it's really annoying temperature sensitive P.O.I. problem.   Probably I'll wait for someone's proven success story before I take a chisel to any part of it.....since if such doesn't solve things, I can't undo it.

That said, I hope to get out this weekend as a last hunt before I stop until the juvenile pests are out (June)....the rancher wants me to be effective, so sadly the .22 HW80k stays home and the .177 Beeman R9/HW95 goes.  BTW, I put a new barrel with a REALLY tight breech on the R9 and I gained est. 30 fps.   Does more of the "pop the cork" now....so power builds up better than a less tight breech.....I suppose I could put the old .177 R9 barrel on the 80k to play with....to see if there is any detent lockup difference that affects the temperature sensitive POI issue.   But for now, I'm in a time bind to get about 4 families' taxes done....so that must wait.   (relatives...one doesn't marry a Chinese wife....one marries a Chinese family 🙂 ).

I have not solved the heat related poi shift problem, but in an effort not to chase wild gooses I would ignore metal expansion until the very last. Keeping in mind that all of your clearances are maintaining themselves cold to hot. All parts are expanding and contracting according to temperature at the same rate, so the relative fitment of the parts are the same as long as the parts are steel.

In the machinery world, where there is a big change in startup to running temps thermal expansion is accounted for by having a bearing that  can float axially in its housing with the other end bearing being fixed.

I think the greatest expansion you would encounter would be in overall length of the action tube, which makes me think it could be a bedding issue, as the expansion rate of wood and steel are much different. Perhaps set up the bedding in the action so the front screws become the fixed end with a pr0vision to transfer recoil to the stock and let the trigger guard screw allow for expansion to the rear. Strange that the pattern is so consistent amongst a variety of rifles???

To follow up on ribbonstone’s and stevevines’s comments, I have discovered that how you hold a spring gun can actually affect muzzle velocity. I laughed when a friend told me this...until I tried it over my own chrono. If you pull the gun tightly into your shoulder, you will consistently get an extra 10 FPS or more, compared to a looser “artillery” hold.

So it’s easy for me to believe that the clothes you wear in cold vs. hot temps could make a measurable difference. 

I will add the caveat that I do not do much long-range precision shooting compared to most of you gents; but it’s hard for me to see how temps could affect barrel lock-up on a break-barrel gun, unless maybe you had some awfully stiff grease on the detent. I would note as back up that Mr. Hamilton had break-barrels on both his “affected” and “not affected” lists...

Hi, this is my first post so hopefully not breaking some newB protocol but I found your thread interesting. One thing that occurred to me that might be helpful is something similar that we have to deal with in auto racing (disclaimer: I'm a hobbyist, not a professional). Air density. Just like the piston in our springers, the piston in an engine relies more on air density (and air pressure) to ingest a full chamber of air (naturally aspirated engine). A difference in temp. between 60 - 75f as you mentioned makes a noticeable difference in a performance vehicle because the air (oxygen) is less dense.

This seems to line up with your experience as I notice the problem seems much less prevalent in the smaller caliber/lighter trajectory/higher power guns that will shoot a flatter trajectory in the ranges you are working with. Whereas the guns with less power/heavier pellets will be more susceptible to POI drop.

Anyway, just a thought. Would be interesting to put a chrono on them at the muzzle and 40 yds out in the two temps just to see. If barrel chrono changes than something with the gun. If stays same but 40 yd changes than something with atmosphere (density, pressure...).

Great forum, lots of knowledge here. Appreciate you guys.

Have looked over a lot of my old notes (and besides on-line, it pays to keep a notebook...or a section of a ring binder...for each airgun...as computer crash/old fourms turn into new forums/etc).

We'll keep "shots out of storage" seperate form "andering zero".
Have had 3 springers that did not use tradtional piston seals: FWB 150. FWB 300, and a Wrbley Osprey al used a kind of piston ring system, so it wasn't a matter of the "soft cup" limbering up as it warmed up from the heat of compression. Those 3 generally took very few "out of storage" shots to get back shooting to where I expected them to be, summer or winter.

Maybe 6 "regular" springers, all relyng on the traditional piston seal at the end of the piston. Going from storage, they all took more shots to get stale, and the stability was often not quite on the past POA. Most never were far off, but measurably off (like maybe 1/4MOA).

Of those 6, a couple of them did eventually get back to the POA if you had enough patience to shoot long enough (like 40-60shots) withouttouching the scope adjustments.

Of those 6, another 3 of them would stabilze thier POI, but NOT their POA, and would need scope readjustment to continue shooting.

One of the 6 never got with the program. Eventually traced down to one of the stock screws that felt tight, but was really bottoming out in it's threaded hole (due to wood stock compression...it felt tight, but wasn't really pulling the stock down tight).  AND, under close exam, there was a slight spint near the rear (trigger guard screw).

Fixed the split with cross dowels/glue.

Could add "cups' to fix the bottomed out side screw, but I just soaked the compressed wood in the bottom of the screw hole and added a washer under the stock screw, which seemed to cure the problem (with no outwardly visible mod).