7mm vs. .30 cal

[RockyMtnMT]

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

 

[Michael Eichele]

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.

 

[mtmuley]

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

 

[Flybuster]

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.

 

[linksmechanic]

One thing to consider with the 7mm is there is better bc bullets out there that more than make up for the shorter velocity.

 

[britz]

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.

 

[Hired Gun]

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.

 

[Pdvdh]

Yes, Hired Gun has this correct.

We just happened to have this same discussion under the subject Stubby--Lapua in the general discussion forum, where we compared 7mm to 338 velocity. But the explanation is the same for the 7mm and 30 cal. Below I've copied and pasted the explanation from the Stubby--Lapua post.

"if equal pressures (say 60,000 psi) are generated in two different cartridges over an equal duration of time, the force exerted on the base of the bullet with the larger diameter will be greater than the force exerted on the base of the smaller diameter bullet. The area across the base of a 7mm bullet is about 0.0634 square inches. The area across the base of a .338 bullet is about 0.0897 square inches. The pressure generated by the cartridge acts against the base of each bullet. In the case of the 7mm bullet, the 60,000 psi creates a force of (60,000 lbs/square inch) X (0.0634 square inch) = <u>3804</u> lbs of force. With the .338 bullet the 60,000 psi pressure creates a force of (60,000 lbs/square inch) X (0.0897 square inch) = <u>5382</u> lbs of force.

We can see that even though the pressure is equal in each bore, much more force is applied against the base of the .338 bullet than the 7mm bullet, because the force exerted is a function of the area that the pressure is able to act on. And I think we can all agree that the greater the force acting on two bullets of equal weight, the faster the bullet will be accelerated down the bore. Which is why bullets of equal weight will always be driven faster in a larger caliber, provided that the same pressures are maintained over the same length of time. So this has a straightforward explanation."

 

[RockyMtnMT]

Thanks guys.

I knew there was a good reason, I just could not figure it out.

So now the question is how far down range the higher bc 7mm bullet has to travel to make up the differance in muzzle velocity. I wonder how that compares w/ same weight bullets, and with say 160grn accubond 7mm ultra compared to 200grn accubond 300 ultra?

Hopefully I'm not beating a dead horse.

Thanks guys, Steve

 

[linksmechanic]

If your dead set on an ultramag stay away from the 7 ultra. Although on paper it sounds like a great idea in reality it's too much of a good thing. This is coming from a 7mm fan. If you are too stick w/ an ulra get a .300. If your looking for someting different try the 7mm stw. Personally I like the 7rm. One other thing: The higher the velocity the shorter the barrel life.

 

[Slopeshunter]

Hi, just out of curiosity and just not knowing much about the RUM's would you mind elaborating a bit for me.

[ QUOTE ]
If your dead set on an ultramag stay away from the 7 ultra. Although on paper it sounds like a great idea in reality it's too much of a good thing.

[/ QUOTE ]

Tks!

 

[RockyMtnMT]

linksmechanic,

I'm not set on anything, I just find this stuff facinating.

Steve