Ok, some more things to ponder

October 4 2011 at 6:03 PM
L D  (Login lhd)

Response to i was out again this morning in "shooting lab"

Springers tend to gain velocity rather suddenly as temps reach more critical higher levels that thin lubes to lower friction further, speeding things up, and this is added to vaporized lube that combusts a bit more easily. Its not a liner process, and all I can say is I have observed springers begin to shoot faster when its hot than when its cold.

BUT, besides that, since aluminum scope tubes "grow" about 2 1/2 times faster than steel receivers, there is a tendency for scopes to be pushed and pulled out of normal alignment.

Yes, I realize folks think the tiny amounts a scope can grow per degree of temp change is inconsequential, but I know better! First off, say a scope grows only .000013" per inch per degree of temp change, and the scope rings are 5" apart at the widest. So, if a gun were zero'd at 65F, and the scope is now 90F, the scope has grown .000325". At the same time, the steel action has only grown around a third of that amount in the area of concern.

BUT, these tiny length changes can cause huge forces that try to bend the scope tube by means of the scope mounts, and there is the potential issue of temperature differentials both within the scope itself and the action, since its not such a hard thing to accept that the action inside the stock cavity might be cooler than the exterior near the scope mounts.

Finally, take into account that WOOD hardly changes in length with small temp shifts, so in effect, it MAGNIFIES any growth seen in the metal action! Yes, wood can grow and shrink a LOT with humidity changes, but if it doesn't get soaking wet, it takes a long time for wood to significantly change its water content, ESPECIALLY to lower it.

I have often seen thin metal that is exposed to the sun bow away from it due to relatively tiny temperature differentials, so I don't get all anal about millionths of an inch coefficients an the like.

For some reason, certain scope models are MUCH more prone to lose accuracy in ranging as temps change. This seems to be much less due to scope materials than scope design itself, since there are scopes made of similar alloys with dissimilar temperature sensitivities. I find the Bushy Elite 6-24 and 8-32 to be darned stable in this respect, and the older Leup Vari-X II 6.5-20x to be one of the worst, and worse yet if "boosted" in power.

My feeling is we don't really need to pinpoint EXACTLY why a gun's impact is shifting with temp changes so much as we need to learn to minimize it and learn to deal with it. Back in the early nineties, I found I could achieve a sliding fit for the scope tube in the front scope mount by slipping o'rings on the clamp screws between the top and bottom ring halves, and setting the screw torque til the scope could just be slide in the mount. The rear mount was set up tight as usual.

Nowadays, I just live with it, and use "mental compensation", since there is no serious degree of accurate ranging to be had by me with my present rig (fixed 10X scope) anyhow. BUT ... my gun and scope don't HAVE so many issues to deal with anyhow.

This message has been edited by lhd on Oct 6, 2011 10:23 PM

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On line trajectory calc by Brad Troyer MILDOTS


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