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Stubby--Lapua
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<blockquote data-quote="Fiftydriver" data-source="post: 158247" data-attributes="member: 10"><p>Bearless,</p><p></p><p>Expansion ratio is a term that compares the capacity of a cartridge to the volume if a specific bore diameter.</p><p></p><p>For instance, and these are just numbers I am throwing out there to show what this is. Say you take a 7mm RUM that has a case volume of say 0.4 in cubed. If you have 24" of actual bore length( around 27" of total barrel length including chamber), the bore volume will be around 1.520 inches cubed.</p><p></p><p>To figure the expansion ratio, you simple figure how many times the case volume will have to multiply to fill the bore volume. In this case it takes 3.8 times the case volume to fill the bore volume.</p><p></p><p>If we take that same capacity and increase the bore diameter to 375 caliber in the case of the 375 RUM. With the same length barrel we have a bore volume around 2.651 inches Cubed. In this case, the case volume will need to expand 10.6 times to fill the bore volume.</p><p></p><p>In the 7mm, the powder volume will not be able to expand nearly as quickly as with the 375 so we need to use a very slow burning powder to maintain proper pressures in the bore without generating extreme pressure spikes.</p><p></p><p>In the 375 example, we need a much faster burning powder so that the powder can burn and expand fast enough to maintain proper pressure applied to the bullet to generate good velocity.</p><p></p><p>If we use to fast of a powder in the 7mm we get into severe pressure problems because the powder will expand at a rate greater then the bore volume will allow.</p><p></p><p>If we use to slow of a powder in the 375 we will not get good velocity because we will not be able to maintain proper pressure in the bore down the entire length of the barrel simply because the slow burning powder can not generate enough gas volume quickly enough to do this.</p><p></p><p>Its all a balance of bore diameter and volume to case capacity and bullet weight, basically sectional density is what we are talking about. The ratio of bullet weight to bore diameter.</p><p></p><p>Good Shooting!!</p><p></p><p>Kirby Allen(50)</p></blockquote><p></p>
[QUOTE="Fiftydriver, post: 158247, member: 10"] Bearless, Expansion ratio is a term that compares the capacity of a cartridge to the volume if a specific bore diameter. For instance, and these are just numbers I am throwing out there to show what this is. Say you take a 7mm RUM that has a case volume of say 0.4 in cubed. If you have 24" of actual bore length( around 27" of total barrel length including chamber), the bore volume will be around 1.520 inches cubed. To figure the expansion ratio, you simple figure how many times the case volume will have to multiply to fill the bore volume. In this case it takes 3.8 times the case volume to fill the bore volume. If we take that same capacity and increase the bore diameter to 375 caliber in the case of the 375 RUM. With the same length barrel we have a bore volume around 2.651 inches Cubed. In this case, the case volume will need to expand 10.6 times to fill the bore volume. In the 7mm, the powder volume will not be able to expand nearly as quickly as with the 375 so we need to use a very slow burning powder to maintain proper pressures in the bore without generating extreme pressure spikes. In the 375 example, we need a much faster burning powder so that the powder can burn and expand fast enough to maintain proper pressure applied to the bullet to generate good velocity. If we use to fast of a powder in the 7mm we get into severe pressure problems because the powder will expand at a rate greater then the bore volume will allow. If we use to slow of a powder in the 375 we will not get good velocity because we will not be able to maintain proper pressure in the bore down the entire length of the barrel simply because the slow burning powder can not generate enough gas volume quickly enough to do this. Its all a balance of bore diameter and volume to case capacity and bullet weight, basically sectional density is what we are talking about. The ratio of bullet weight to bore diameter. Good Shooting!! Kirby Allen(50) [/QUOTE]
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