Temperature effecting POI

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TiredRooster

TiredRooster

Member
Nov 1, 2018
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Alabama, United States
I think I may have just stumbled across something new to me. I have been enjoying shooting my D34 classic with the new Vortex 4-12. The rifle has been a tac driver. This evening I get home from work and shoot a bit just before dark and the groups are very tight still, but are about one inch low and a bit left (this is at 25 yards). I make a couple of quick scope adjustments and get the pellets punching holes right where they are supposed to and with consistent tight groups still. The only variable that changed is the temperature. The day it was originally sited in and everyday of shooting since has been 60-70 degrees F. This evening it was about 40. I can only assume that the 20-30 degree cool off effected my POI somehow. Possibly by way of slowing velocity. I don't know, didn't chrono. Any body concur with my theory or experience the same with a springer? 
 
Decades ago when shooting early spring hunter class field target with my .177 R9 (near Harrisonburg, VA and club no longer exists) the sightin was done during snow flurries, however the temp rose to upper 50s during the match and my poi shifted 1" at only 20 yards. My brother was also shooting the same match with his R9 and had the exact same poi shift with the temp rise, however my poi shift was low right and his was high left.

Thinking that perhaps the viscosity of the lube used and the durometer of the piston seal changed which affected the functioning of the piston. To test the theory I stripped the gun of all existing internal lubes and replaced them with sparingly applied molly paste, even for the spring & guides. I then went shooting when the temps changed about 25 degrees during a WV squirrel hunt and found that the poi shifting was a lot less than when I had "tar" on my spring, however poi shifting was still evident.

That gave me the idea to minimize the amount of "rubber" sliding on the receiver ID by cutting an aluminum oring sealed piston cap with a 1/16" nominal cross section oring I bought at an ACE home supply store. The cap looked similar to this............



When testing out the oring sealed piston cap I found a couple benefits, the temp related poi shifting was GREATLY reduced and instead of requiring a bunch of "warm up shots" to stabilize the poi I only needed a couple. That was over 30 years ago and I'm still using home turned oring sealed piston caps in my HW springers.

Anywhoo.....a 25 degree temp shift did cause a 1" @ 20 yard poi shift with my .177 R9!
 
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Decades ago when shooting early spring hunter class field target with my .177 R9 (near Harrisonburg, VA and club no longer exists) the sightin was done during snow flurries, however the temp rose to upper 50s during the match and my poi shifted 1" at only 20 yards. My brother was also shooting the same match with his R9 and had the exact same poi shift with the temp rise, however my poi shift was low right and his was high left.

Thinking that perhaps the viscosity of the lube used and the durometer of the piston seal changed which affected the functioning of the piston. To test the theory I stripped the gun of all existing internal lubes and replaced them with sparingly applied molly paste, even for the spring & guides. I then went shooting when the temps changed about 25 degrees during a WV squirrel hunt and found that the poi shifting was a lot less than when I had "tar" on my spring, however poi shifting was still evident.

That gave me the idea to minimize the amount of "rubber" sliding on the receiver ID by cutting an aluminum oring sealed piston cap with a 1/16" nominal cross section oring I bought at an ACE home supply store. The cap looked similar to this............



When testing out the oring sealed piston cap I found a couple benefits, the temp related poi shifting was GREATLY reduced and instead of requiring a bunch of "warm up shots" to stabilize the poi I only needed a couple. That was over 30 years ago and I'm still using home turned oring sealed piston caps in my HW springers.

Anywhoo.....a 25 degree temp shift did cause a 1" @ 20 yard poi shift with my .177 R9!
Click to expand...



nced, Thank you for the interesting and informative response. I thought it was most likely the large temp drop that changed my POI. My 34 is well lubed, but lightly, inside with moly paste, no heavy tar. I feel better now knowing that . That was a pretty clever idea making the o-ring seal cap. Is it durable and maintain consistent velocities?

Thanks

TR




 
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My 34 does that a bit too. In AZ the temp can range a lot from the middle of the day to sunset, my POI will shift in a sort of an 8 O-clock to 2 O-clock angle on the bulls-eye, just a little, couple of clicks and it back on target.
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Thanks greenterror.

I'm continuously learning things with this 34. Good to have others that can share experiences too.
 
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How are your scope mounts setup. Sometimes, the poi shift can happen with two piece mounts. It is possible to setup two piece mounts that are resistant to poi/temperature shifts, but easier to do with one piece.

When temperature changes, is the poi shift scope related? velocity related? or both? When you experience the shift, you NEED to chrony it. Otherwise you are just guessing, and the solution will be somewhat elusive.
 
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Mounts are one piece BKL mounts. They don't move. This is first POI shift I have experienced with this rifle/scope setup. It also is the first time I have shot this particular rifle in the colder temps. I could not chrony it because it got dark outside or I would have. After the scope adjustment, it was dead on with consistency. Tomorrow morning will be in the 20's and I'll be off work and will have the opportunity to see how it does then.
 
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Mounts are one piece BKL mounts. They don't move. This is first POI shift I have experienced with this rifle/scope setup. It also is the first time I have shot this particular rifle in the colder temps. I could not chrony it because it got dark outside or I would have. After the scope adjustment, it was dead on with consistency. Tomorrow morning will be in the 20's and I'll be off work and will have the opportunity to see how it does then.
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???

Did you mean: "The mount is a one piece BKL mount. It doesn't move." ?
 
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Mounts are one piece BKL mounts. They don't move. This is first POI shift I have experienced with this rifle/scope setup. It also is the first time I have shot this particular rifle in the colder temps. I could not chrony it because it got dark outside or I would have. After the scope adjustment, it was dead on with consistency. Tomorrow morning will be in the 20's and I'll be off work and will have the opportunity to see how it does then.
Click to expand...



???

Did you mean: "The mount is a one piece BKL mount. It doesn't move." ?
Click to expand...



Correct...IT doesn't move. I have more than one BKL mount on more than one rifle. THEY have never moved.
 
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You've had better luck with one piece BKLs than I had on my .177 Beeman R9 or .177 HW95.

Using less than 15in/lbs torque for the top straps and 25in/lbs with the base screws..........



Increased the base screw torque to 35in/lbs. Solved the scopes shifting in the BKL rings by lining the rings with cloth electrical friction tape..........



My BKLs have been removed..........



They were replaced with universal dovetail to Picatinny adapters with cheap (under $10 at "WallyMart") Weaver Quad Lock rings and all shifting issues were/are solved. The universal adapter has a one piece base similar to the BKL, however they also include stop pins...........









LOL.....even the scope height above the receiver with the adapter and Weaver rings is good when using a scope with a 50mm objective..........





Anywhoo......this reply is for those that have had "slipping issues" with BKL mounts like myself, there is an alternative that will work!


 
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Thats interesting. On my other D34 I have easily 1,500+ firing cycles on the one piece BKL mounts and have experienced NO movement of either the base or the scope in the rings. I really haven't checked for that on this 34. But, I spent the day today walking our property and the rifle/scope is dead on still after the adjustments made yesterday evening. The temps never made it out of the 40's today. I'm pretty well convinced the POI change was temperature induced. If I eventually find out other wise I'll let ya know.

Thanks for all the input.

TR
 
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Last year I sent my Prosport into PA for some type of issue that I forget. Well anyway upon returning, unknown to me, PA heavy tarred my spring and honestly it sounded beautiful because there was no sound, twang or whatever you want to call it. Later, about 2 weeks give or take, I participated in a FT (HFT) at DCFS here in SE Pennsylvania. It was a little chilly in the morning and I was kicking butt, but the temp by late morning/early afternoon had risen about 15 degrees and my POI was at least 1 inch from my POA. Hope that helps. One thing for certain, two things for sure, that Prosport was stripped and cleaned and went back to normal.
 
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Last year I sent my Prosport into PA for some type of issue that I forget. Well anyway upon returning, unknown to me, PA heavy tarred my spring and honestly it sounded beautiful because there was no sound, twang or whatever you want to call it. Later, about 2 weeks give or take, I participated in a FT (HFT) at DCFS here in SE Pennsylvania. It was a little chilly in the morning and I was kicking butt, but the temp by late morning/early afternoon had risen about 15 degrees and my POI was at least 1 inch from my POA. Hope that helps. One thing for certain, two things for sure, that Prosport was stripped and cleaned and went back to normal.
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Josh,

I don't have any heavy tar anywhere in my 34. Its lubed relatively lightly with moly paste on piston/spring and a light brushing of clear tar on the spring guide. However, I am pretty well convinced that the temp swing is the catalyst behind my POI shift. I got a little bit of a laugh...make it a chuckle, outa your story. I tell ya...I think I would have been p.o.'d a bit at PA tarring up my spring unbeknownst to me and it messed my field target event up for me. Anyway....I bet you did clean it up first thing. I would have too...LOL

Thank you.
 
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