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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Caution in the cold
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<blockquote data-quote="David Urasky" data-source="post: 1564324" data-attributes="member: 108550"><p>I find this idea interesting so I did some digging in my Machinery's Handbook (the machinist's bible) and applied some laws of Physics.</p><p></p><p>Coefficients of thermal expansion is how much the material will expand or contract with temperature change. It's measured in units of linear expansion per unit of length per Deg. F.</p><p></p><p>Carbon Steel 0.00000633"/1" length/degree F</p><p></p><p>Before you get all crazy about your barrel material carbon steel is mostly iron, about 98-99% iron. 4100 series steels (chrome moly) is about 98% iron. 400 series stainless is about 85% iron. So they are all pretty similar for iron content. Since I don't have thermal expansion rates handy for all alloy types iron will have to do.</p><p></p><p>So your barrel has an average diameter of 1" for ease of numbers and you're shooting a 308. So the outside of the barrel will contract 0.00000633*1" diameter of barrel*(+80 deg to -30= 110 change) = .0006963". But the outside of the barrel means nothing to the bullet. The bore, which contacts the bullet, would contract .00000633*.308*110=.0002144" Now let's remember that.</p><p></p><p>The bullet, mostly lead with a thin jacket which is made to deform, has a coefficient of thermal expansion of 0.0000163. So your bullet will contract .0000163*.308*110=.00068134"</p><p></p><p>So, your barrel shrinks .0002144" while the bullet shrinks .00068134" meaning your bullet shrinks .000466" more than the barrel. So it's looser in the barrel!!! Should be lower pressures <u>for the first round</u>. After that, your barrel temperature has risen from each previous shot meaning it gets bigger but the bullet is still the same size … unless you leave it in a warm chamber and it has the chance to heat up. And, as the bullet travels down the barrel it heats from hot gasses behind it and friction consequently it gets bigger the further it goes down the barrel which happens no matter what the temperature is. So the bullet is tighter in the barrel the further down the barrel because the bullet is heating but the barrel is still cold in front of the bullet.</p><p></p><p>So based on all of that mess, I don't think any of that's worth understanding. But I found it sort of fun. I believe it comes down to powder temperature. Which is why in 1000 yard F class competition we will place the cartridge into the action, but not close the bolt until you've figured your hold from the previous shot. Then close the bolt that put the cartridge into a hot chamber and shoot. Don't ***** foot around and let the powder heat up or burn rate WILL change as will point of impact. And when trying to hit a 5" X ring at 1000 yards you need to be consistent.</p><p></p><p>Since I've seen point of impact change because of powder temperature increasing, it stands to reason that it will also change when the temperature drops. Which is why I have different loads, in fact different powder, for my 20 Tactical for shooting coyotes in the winter and P-dogs in the summer. Coyotes one shot, one kill, cold barrel, cold ammo. P-dogs 80+ degrees hot sun, hot ammo and way hot barrel from multiple shots.</p><p></p><p>Now wasn't that fun?</p></blockquote><p></p>
[QUOTE="David Urasky, post: 1564324, member: 108550"] I find this idea interesting so I did some digging in my Machinery’s Handbook (the machinist’s bible) and applied some laws of Physics. Coefficients of thermal expansion is how much the material will expand or contract with temperature change. It's measured in units of linear expansion per unit of length per Deg. F. Carbon Steel 0.00000633”/1” length/degree F Before you get all crazy about your barrel material carbon steel is mostly iron, about 98-99% iron. 4100 series steels (chrome moly) is about 98% iron. 400 series stainless is about 85% iron. So they are all pretty similar for iron content. Since I don’t have thermal expansion rates handy for all alloy types iron will have to do. So your barrel has an average diameter of 1” for ease of numbers and you’re shooting a 308. So the outside of the barrel will contract 0.00000633*1” diameter of barrel*(+80 deg to -30= 110 change) = .0006963”. But the outside of the barrel means nothing to the bullet. The bore, which contacts the bullet, would contract .00000633*.308*110=.0002144” Now let’s remember that. The bullet, mostly lead with a thin jacket which is made to deform, has a coefficient of thermal expansion of 0.0000163. So your bullet will contract .0000163*.308*110=.00068134” So, your barrel shrinks .0002144” while the bullet shrinks .00068134” meaning your bullet shrinks .000466” more than the barrel. So it’s looser in the barrel!!! Should be lower pressures [U]for the first round[/U]. After that, your barrel temperature has risen from each previous shot meaning it gets bigger but the bullet is still the same size … unless you leave it in a warm chamber and it has the chance to heat up. And, as the bullet travels down the barrel it heats from hot gasses behind it and friction consequently it gets bigger the further it goes down the barrel which happens no matter what the temperature is. So the bullet is tighter in the barrel the further down the barrel because the bullet is heating but the barrel is still cold in front of the bullet. So based on all of that mess, I don’t think any of that's worth understanding. But I found it sort of fun. I believe it comes down to powder temperature. Which is why in 1000 yard F class competition we will place the cartridge into the action, but not close the bolt until you’ve figured your hold from the previous shot. Then close the bolt that put the cartridge into a hot chamber and shoot. Don’t ***** foot around and let the powder heat up or burn rate WILL change as will point of impact. And when trying to hit a 5” X ring at 1000 yards you need to be consistent. Since I've seen point of impact change because of powder temperature increasing, it stands to reason that it will also change when the temperature drops. Which is why I have different loads, in fact different powder, for my 20 Tactical for shooting coyotes in the winter and P-dogs in the summer. Coyotes one shot, one kill, cold barrel, cold ammo. P-dogs 80+ degrees hot sun, hot ammo and way hot barrel from multiple shots. Now wasn't that fun? [/QUOTE]
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