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7mm vs. .30 cal

 
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  #1  
Old 06-23-2007, 06:55 AM
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7mm vs. .30 cal

Hi all.

My question has probably been addressed before, but I need help understanding this.

Why does a 30cal bullet seem to achive greater velocity than a 7mm? To compare fairly lets compare 7mm ultra to a 300 ultra. If I am not mistaking they are the same case. Why is it that the 7mm preassures out w/ the same weight bullet before it can reach the same velocity acheived w/ the 300 ultra?

Thank you all for the help in advance. Steve
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Old 06-23-2007, 09:35 AM
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Re: 7mm vs. .30 cal

A 30 cal built on the same 7mm case with the same barrel length will typically get better velocity due to burn area. A 30 cal hole is bigger. It is a simialr principal to adding length to a barrel. More burn area, more velocity.
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Old 06-23-2007, 07:39 PM
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Re: 7mm vs. .30 cal

The .300 Ultra seems to be a cartridge that is extremely efficient. For the amount of powder it burns, it seems to turn it into max velocity and energy in any weight bullet. Get one. mtmuley
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Old 06-24-2007, 04:18 PM
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Re: 7mm vs. .30 cal

If the barrel length, twist rate, powder charge and bullet weight are the same for both calibers the muzzle velocity will be higher for the wider bore. There is more surface for the gasses to push against with a 30 caliber than a 28 caliber. But the 28 caliber will also have the higher Ballistic Coefficeint so it will retain velocity better than the comparable 30 caliber.
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Old 06-24-2007, 05:55 PM
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Re: 7mm vs. .30 cal

One thing to consider with the 7mm is there is better bc bullets out there that more than make up for the shorter velocity.
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Old 06-24-2007, 07:32 PM
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Re: 7mm vs. .30 cal

I read a post from a person who claimed to be a enginere who seemed to make sence (Under Stubby Lapua article. His basic point was that given the same pressure, same weight and different bullet diameter, the force presented on the bullet will net greater on the larger diameter. THink about it, Pounds per square inch. a .308 will net more pounds per square inch than the .284.
However, keep in mind, larger diameter, same weight will net lower BC and do poorer down range.
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Old 06-24-2007, 08:03 PM
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Re: 7mm vs. .30 cal

I don't know that I would call the 300 Ultra efficient at all. I can run about right with it with 10 grain less powder with my 300Wby. I shoot 180's at 3300 with 85 grain of powder and 200 grain Accubonds at 3100 with 83 grains. No extractor marks and 20 plus firings for case life.

The reason the bigger bore gets it's velocity easier with the same weight bullet is the bottom of the bullet has more area to push on and has less weight per square inch of area. A 308 bullet base area is .0744682 square inches. A 7mm has and area of .0633149 sqaure inches. With the same amount of pressure acting on the bottom of the bullets base the bigger one will support more weight. A larger diameter piston makes more power. Same reason NHRA Pro Stock engines have a very large bores.

I started this post this morning but had to go shooting. Sorry for the delay.
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