tempature and point of impact and powder?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by haeffnkr, Oct 11, 2004.

  1. haeffnkr

    haeffnkr Member

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    What effect does tempature have on point of impact?
    If a gun is stable and sighted in and the load is stable and it is summer (not blazing hot) then the same gun and load is left to sit in 30 degree windy overcast temps for an hour or 2 will the POI move? If so how much?

    BTW -I am assuming the gun has been properly bedded and nothing is warping on the gun.

    thanks Kevin
     
  2. Brien

    Brien Well-Known Member

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    colder air is heavier air (should require more elevation)... colder weather also slows the burn rate down .... we shot dad's 7ultra in july 80+ degrees = 3280--- in january +8degrees (fht)=3120 to 3150 fps...temp drop 10degrees from one day to the next can change a zero a couple clicks
    Brien
     
  3. budlight

    budlight Well-Known Member

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    For my fall and winter hunting I always keep my bullets in an inside pocket so body heat might keep them near 80 or so.

    I also only load a single round just before shooting. I use all SPBT's and if you load the magazine it just flattens the tips.

    I have hunted in way below "zero" F. with consistant shots
     
  4. Gun5tuff

    Gun5tuff Member

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    Haeffnkr has raised a point that should be addressed by powder manufacturers and made available to hunters.

    I was developing a load to fit the ballistics of my scope and wanted 2750 ft/sec from a 180gr Hornady BTSP in a .30-06 case. I used 56.0 gr of H4350. At 95 degrees F the load gave me 2822 +/- 8 ft/sec while at 33 degrees F the same load gave me 2752 +/- 9 ft/sec for a 6-round average. I am happy with this load but I still need to test it at long range to see if it actually does fit the published ballistics of my scope. The main point of this thread, however, is the change in bullet velocity as a function of temperature. For all practical purposes (and hunting is such a purpose)it isn't anything to worry about unless you want to be prepared to take a long shot with greater confidence.Haeffnkr
     
  5. milanuk

    milanuk Well-Known Member

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    Kind of surprising that you have that much variation w/ H4350; it is supposed to be one of Hodgdon's 'Extreme' powders produced specifically to be temperature stable. AFAIK, Varget, H4350, H4831SC, and H1000 are all supposed to be. Varget I can vouch for, is pretty dang temperature stable. Not completely unaffected, but noticeably less so (in my guns) than say, the Alliant Reloader series, or other such as IMR 4064.

    Vihtavouri powders are supposed to be kind of middle of the road, Ramshot powders are specifically billed as being temperature stable (though more than one person has determined that Tac is less stable than say, Varget, but comparable to RE-15).

    I believe more manufacturers are making some sort of effort to minimize temperature sensitivity of their powders as that is what shooters today expect and are wanting. Like I said, generally speaking Hodgdon Extreme and Ramshot powders are more so than most. Even so, to be responsible about it, if you want to play around shooting at long range in widely varying temperatures, you need to test your loads under said conditions. Take your loads to the range in a cooler w/ an ice pack in it in the summer, and see what a chilled round does in your gun. Similarly, take some rounds to the range in the winter, stick them in a snow drift, and then shoot them. Take some others, and set them over the defrost vent in your rig, and heat them up to 90+F, and shoot them. Check the velocity of each. All kinds of data you can accumulate if you want to spend the time [​IMG]

    Monte
     
  6. Mike in Texas

    Mike in Texas Well-Known Member

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    Try logging the barometric pressure when you chronograph.

    You will find that when you see a 0.1 change
    in the absolute or station pressure you will see a corresponding shift in impact.

    Local pressure (absolute) has a greater effect on impact than temperature in general terms.

    A Suunto Vector or Casio Triple Sensor watch is an invaluable tool.
     
  7. Mysticplayer

    Mysticplayer Writers Guild

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    Changes in temp will affect your shooting two ways: your powder and load pressure, and your barrel.

    I use the Hodgdon Extreme line of powders and have not experieced any measurable difference in my drop table from the summer (up to 35C or 100F) down to fall hunting which can hover above freezing.

    If you use any other powder, you could see up to 200fps difference over that temp change. That not only changes your POI (each rifle will be different) but your accuracy may go for a dump too.

    The second way is how your barrel changes in the cold. Usually, there is little affect when temps are above freezing, however, when temps plummet, steel can change.

    The only way to know is to shoot in the low temps. My otherwise superbly stable '06 shot low when subjected to -15C temps. That would have affected any LR doping. After a few shots, the POI returned to 'zero'.

    Every load and rifle should be tested as much as possible under the conditions you will be hunting in. Failing that, the first shot sighter tech will help.

    Jerry
     
  8. Brent

    Brent Well-Known Member

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    I'll add to what Budlight was saying, in that the rifle temp, air temp and BP are the variables if you keep your ammo a consistant temp.

    30 degrees could mean a 3/4 MOA POI shift possibly. You will only "know" from logging the data and examining it though.

    To throw a wrench in it... you might find that POI can actually be higher with a cooler temp, if your ammo is allowed to stabilize at the lower temp.

    I think it is just easier to completely eliminate the ammo temp / MV / barrel harmonics variable. Left are the temp, BP, wind, range and angle variables, which are all pretty easily dealt with though.

    If the sun is beating down on one side of your barrel in the morning, then the other side in the afternoon, how much left-right POI shift will result if ambient temp was +100 deg F? What if the sun is directly overhead, but then the clouds roll in and the barrel then evens out in temperature with the underside?