Coyote shooters, does -15 cold affect POI
- Thread starter coach71
- Start date
The ammunition itself might be temperature sensitive. I wondered about this a couple of years ago with my deer hunting rifle (Tikka T3 in 308 Win), so I took it to the range and let is sit out for half an hour in the cold before I fired a shot. Propellant was IMR4895, and it didn't print any different on the 1st shot than it had previously been sighted in to.
Actual testing gives you the best peace of mind.
Actual testing gives you the best peace of mind.
Aluminum bases and rings will contract more than steel and can cause the scope to bend slightly. Tapered bases will contract unevenly and change your POI.
Even non-tapered bases can have a slight taper due to manufacturing tolerances. Same thing with rings. Also, evening out the rings with a ring-reaming setup may be a good thing.
You can use calipers to measure the thickness between the bottom of the ring and the bottom of the base - take the rings off the scope and the base off the rifle first obviously Not the different front to rear and shim accordingly (with the same metal as the rings and base are contructed of).
Ammo being cold can change POI. If it's -30 or so, I usually keep ammo in my inside pocket - I wear a layer over my pants with the pocket cut out so I can reach through - to keep it warmer.
Cold weather causes screws fastening the action to the stock to contract that can put upwards pressure on the barrel also. Is you barrel floated? If so, than check it will the $5 bill when it's -15 - it may be binding.
Pillar bedded rifles put together with steel screws can loosen up in really cold weather. I put acraglass on my pillar bedded rifle. Seems to help. I didn't notice it until I pulled the stock off after the gun had been in the car at -35. They were barely torqued.
Even non-tapered bases can have a slight taper due to manufacturing tolerances. Same thing with rings. Also, evening out the rings with a ring-reaming setup may be a good thing.
You can use calipers to measure the thickness between the bottom of the ring and the bottom of the base - take the rings off the scope and the base off the rifle first obviously Not the different front to rear and shim accordingly (with the same metal as the rings and base are contructed of).
Ammo being cold can change POI. If it's -30 or so, I usually keep ammo in my inside pocket - I wear a layer over my pants with the pocket cut out so I can reach through - to keep it warmer.
Cold weather causes screws fastening the action to the stock to contract that can put upwards pressure on the barrel also. Is you barrel floated? If so, than check it will the $5 bill when it's -15 - it may be binding.
Pillar bedded rifles put together with steel screws can loosen up in really cold weather. I put acraglass on my pillar bedded rifle. Seems to help. I didn't notice it until I pulled the stock off after the gun had been in the car at -35. They were barely torqued.
No.
from 20 c to -15 yes, will depend on what temp you zeroed at. Powder burns slightly slower, harder to ignite, air is more dense. Contracting bore would result in higher pressure and velocity, could move you bullet either way depending on sight height and distance fired, also would depend on the differential of bore/bullet contraction.
2.5 moa @ 600 yards with my 6.5 -284 with the august zero in late october....
Easy to think way to hard about this just re zero at around the ambiant temp you will be shooting at, also should reshoot different distances to make sure your range/ bullet drop data is still good.
Andy
2.5 moa @ 600 yards with my 6.5 -284 with the august zero in late october....
Easy to think way to hard about this just re zero at around the ambiant temp you will be shooting at, also should reshoot different distances to make sure your range/ bullet drop data is still good.
Andy
- Location
- Prince Albert, Sk
I just looked through my Sierra program, a 40 degree swing, -20 to +20 was a difference of under 2" @ 500 yards, vel of 1982 to 1904(for one of my loads). So as for air density changes, you will still hit the dog and not have to calculate or worry about variables to me. Sight in at 0 degrees and don't sweat the details. 
Here's what once happened to me. Was out deer hunting in about -20 degees temp. Shot at a 3pt buck at about 75 yds, perfect rest and all, deer not moving. Aimed at neck, and hit the front leg at about the knee. Deer just stood on 3 legs, so I proceeded to empty out the clip aiming at the chest, all clean misses. Reloaded and finally hit chest with about the 7th shot.
Both before and after this event, the gun shot flawlessly with great accuracy. However, that was the only time I was out in such cold weather.
Don't own the gun anymore.
Both before and after this event, the gun shot flawlessly with great accuracy. However, that was the only time I was out in such cold weather.
Don't own the gun anymore.
Where did you get this information?? Expansion rate5x that of normal steel? What exactly is normal steel? There are hundreds of types of both stainless steels and "normal" steels. This statement means absolutely nothing....
Check your facts before making crazy statements like that. Mild steel has an expansion coefficient of about 13 millions of an inch per inch per degree F. SS varies from about 9 to 17. Aluminum is around 22. I don't have data but I don't see why a chromolly steel wouldn't be in the same ballpark as SS or mild steel. Your lead bullets have a coefficient of about 28.
Plastics vary from about 50 to 200 depending on the polymer. Maybe that's what you were thinking of. Composite stocks can vary from zero to much greater than that of steel.
Wood has a very low expansion coefficient - varying from about 3 to 5 (at least for common hardwoods). Makes sense, if it didn't trees wouldn't survive Canadian winters
As far as doing the math and figuring out what it all means to your rifle accuracy, I suggest you just test fire it
I am no expert, but I did do OK in chemistry.Also did a project on thermal expansion rate of stainless steel vs conventional steel rifle barrels just last year, testing 6 rifles. Three different models, one being SS, the one conventional of each. (it was saved on my old computer,so this is the best I can do).
The coefficient of thermal expansion is expressed in units of proportional change of length for each degree increase in temperature, usually x10-6/°C, μm/m/°C, or x10-6cm/cm/°C, all of which are identical units. The increase in length (or diameter, thickness, etc) can be readily calculated by multiplying the original dimension by the temperature change by the coefficient of thermal expansion. For example, if a three metre long Grade 304 bar (coefficient of expansion 17.2 μm/m/°C) is heated from 20°C to 200°C, the length increases by:
3.00 x 180 x 17.2 = 9288 ÎĽm = 9.3 mm
The coefficient of thermal expansion of the austenitic stainless steels is higher than for most other grades of steel. (tests on rifle barrels the expansion rates varied up to 5X between stainless, and conventional steel depending on temperature)
This expansion coefficient not only varies between steel grades, it also increases slightly with temperature(stainless barrel vs conventional steel). Grade 304 has a coefficient of 17.2 x 10-6/°C over the temperature range 0 to 100°C, but increases to a greater extent above/below these temperatures
The effect of thermal expansion is most noticeable where components are restrained, as the expansion results in buckling and bending. A problem can also arise if two dissimilar metals are fabricated together and then heated( barrel, and action). Dissimilar coefficients will again result in buckling or bending. In general the quite high thermal expansion rates of the austenitic stainless steels mean that fabrications in these alloys may have more dimensional problems than similar fabrications in carbon or low alloy steels, in ferritic, martensitic or duplex stainless steels.
The coefficient of thermal expansion is expressed in units of proportional change of length for each degree increase in temperature, usually x10-6/°C, μm/m/°C, or x10-6cm/cm/°C, all of which are identical units. The increase in length (or diameter, thickness, etc) can be readily calculated by multiplying the original dimension by the temperature change by the coefficient of thermal expansion. For example, if a three metre long Grade 304 bar (coefficient of expansion 17.2 μm/m/°C) is heated from 20°C to 200°C, the length increases by:
3.00 x 180 x 17.2 = 9288 ÎĽm = 9.3 mm
The coefficient of thermal expansion of the austenitic stainless steels is higher than for most other grades of steel. (tests on rifle barrels the expansion rates varied up to 5X between stainless, and conventional steel depending on temperature)
This expansion coefficient not only varies between steel grades, it also increases slightly with temperature(stainless barrel vs conventional steel). Grade 304 has a coefficient of 17.2 x 10-6/°C over the temperature range 0 to 100°C, but increases to a greater extent above/below these temperatures
The effect of thermal expansion is most noticeable where components are restrained, as the expansion results in buckling and bending. A problem can also arise if two dissimilar metals are fabricated together and then heated( barrel, and action). Dissimilar coefficients will again result in buckling or bending. In general the quite high thermal expansion rates of the austenitic stainless steels mean that fabrications in these alloys may have more dimensional problems than similar fabrications in carbon or low alloy steels, in ferritic, martensitic or duplex stainless steels.
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@captonion: I agree with almost everything you said - except for the "5x" statement. Yes, austenitic SS has a coefficient of about 16 or 17. Ferritic SS has a coefficient of about 10. That's a factor of 1.5. 4340 has a coefficient of about 12. So it's "higher" than any other grade of steel I can find data for. Not 5x higher.
It may be that due to the effects of dissimilar metals, the effect on accuracy can be greater than for a rifle constructed of all the same material, depending on how the rifle is constructed. That I can't prove or disprove, except to note that the difference in the coefficient between the stock and the barrel is much greater than the difference between the barrel and the action. This is where I would speculate that a bedded rifle barrel (of any material) would be affected much more than a free floating barrel.
Note that the coefficient of expansion of SS is much closer to the coefficient for aluminum (which is what most scope tubes are made of). So, if your scope is mounted on a SS action you will have less effect on that part of the "shooting system" due to extreme temperature changes. Granted it's only acting over the length of the scope mount so it takes a big temperature swing to have much movement.
Again, test your rifle.
It may be that due to the effects of dissimilar metals, the effect on accuracy can be greater than for a rifle constructed of all the same material, depending on how the rifle is constructed. That I can't prove or disprove, except to note that the difference in the coefficient between the stock and the barrel is much greater than the difference between the barrel and the action. This is where I would speculate that a bedded rifle barrel (of any material) would be affected much more than a free floating barrel.
Note that the coefficient of expansion of SS is much closer to the coefficient for aluminum (which is what most scope tubes are made of). So, if your scope is mounted on a SS action you will have less effect on that part of the "shooting system" due to extreme temperature changes. Granted it's only acting over the length of the scope mount so it takes a big temperature swing to have much movement.
Again, test your rifle.
Mount Sweetness
CGN Ultra frequent flyer
- Location
- Southern Ontario
Put the crosshair on fur and pull the trigger
Every rifle will react differently. You won't know till you test yours. I have an issue of North American Whitetail magazine with an article on this topic. The author's custom made wood stocked bolt action showed a 43 inch POI shift at 100 yards from 32 F to -25 F . That's probably an extreme case, but an example of what can happen.
Shooter error at cold temps is probably a bigger problem.
I've tested more than a few rifles with loads worked up at 20+ degrees in the summer and then shot the same loads with a frozen rifle and frozen rounds at -25 or -30.
Proper bedding has a bigger impact than anything else. Glass bedded stuff is less likely to move significantly in my experience, and synthetic less so than walnut.
Another thing that guys don't really think of, is that if you have a bit of snow or whatnot along your barrel and torch off a couple of shot at several thousand degrees and it melts and slips under your barrel and refreezes that can have an effect on your POI too.
I've tested more than a few rifles with loads worked up at 20+ degrees in the summer and then shot the same loads with a frozen rifle and frozen rounds at -25 or -30.
Proper bedding has a bigger impact than anything else. Glass bedded stuff is less likely to move significantly in my experience, and synthetic less so than walnut.
Another thing that guys don't really think of, is that if you have a bit of snow or whatnot along your barrel and torch off a couple of shot at several thousand degrees and it melts and slips under your barrel and refreezes that can have an effect on your POI too.
bearkilr
CGN Ultra frequent flyer
- Location
- Nowhere, ......just past the middle
Did you miss one recently?
I've never found POI to change significantly enough to matter and that's with the rifle slung over my shoulder on a snowmobile, -30 at times.
