Why is the 25-06 slower then the 30-06 with the same Bullet weight?

[Nimrod1203]

So i was looking through my reloading manual today. and with the 110 or 115 Gr bullet weights, the 25-06 has about 200 FPS slower speed then the 30-06. and about 100 FPS then the 270....i found this VERY odd, seeing how it's the same case. My question is does the smaller opening restrict how much velocity you can get due to pressures? also, the 30-06 was tested with a 24 inch barrel, and the 25-06 and 270 a 26". any insight would be great. i just built a custom 25-06 for out to 600 yards. Thank you.
Nimrod

 

[Browninglover1]

The difference in velocity does have to do with pressures. A 115 grain bullet in a 25 caliber is going to be lots longer and have much more bearing surface that produces friction than a 115 grain bullet from a 30 caliber. The more friction that is created causes higher pressures and does not allow you to achieve the same velocities with the same weight bullets in your example.

That being said, the 25 caliber bullet will have a much higher BC and should fly much better at longer ranges than the 115 grain 30 caliber bullet.

 

[SpencerSS]

Same reason all larger calibers beat smaller calibars on the same case. Ex: 338-08 vs 308.

 

[300WSMMAD]

My understanding on this matter is, in leymans terms.

Its to do with the surface area on the back of the bullet, the more surface area there is for the energy(the bang) to push on, ie the larger diameter bullet receives more energy than the smaller one, so goes faster.

Make sense?

Im no rocket scientist, but thats how I understand it works.

Im more than likely wrong, and apoligise for confusing you if I am.
Until someone confirms Im right, or on the right track,dont take it as gospel truth, I failed Science,Phyics school in general

 

[royinidaho]

Its to do with the surface area on the back of the bullet, the more surface area there is for the energy(the bang) to push on, ie the larger diameter bullet receives more energy than the smaller one, so goes faster.

Make sense?

Your understanding won't lead anyone astray.

Start with the '06 then the 280, 270, 6.5 and 25-06 and pick the same weight bullet in each caliber loaded to the same velocity. As you go down in caliber it will take less powder to achieve the same velocity, correct?

As the bore gets smaller barrel volume decreases (expansion ratio) and bearing surface increases (bullet friction) plus the affects of the smaller surface of the bullet base.

There's probably a very long equation that explains the stuff, but I'm with the common sense approach. It is what it is, and I can live with it

 

[Nimrod1203]

Thank you for the explination. I was really hoping to be able to push the 115 to 3200-3250, but it looks like i'll have to be dancing around the 3050-3100 range. I still think i wanna shoot the 115. It might be the 110 would be the best balance between velocity and BC. i've got a 26" shillen barrel on a tikka action. What kinda Velocity should i see with the 115?
Nimrod

 

[royinidaho]

Shoot the highest bc bullet that your rifle likes best.

Don't be concerned about MV but get as much as you accurately can, then forget about it and go to engaging targets i.e., meat in the freezer.

The higher bc will make for more consistent accuracy in wind. The drop chart will remain constant, along with accuracy. That's all you need.

A few FPS doesn't mean squat to the target engaged. Hitting it does!

 

[6.5Express]

I got over 3100 in my factory kimber 2506 with H1000 1 grain below bergers max with no pressure signs whatsoever. I backed down for accuracy, es, and sd and settled with 56.5 grains H1000 at 3080fps. This is with the 115 vld. You could probably get 3200 in some rifles, whether or not that's where accuracy, es, and sd are best is the question. Velocity sure helps but is not most important in my opinion.
gun)

 

[6.5Express]

110 grain accubonds also shot well in my factory rifle, my preference is the vld.
gun)

 

[danj]

What load were you using for the 110 acubond?

 

[6.5Express]

mine was with 50 gr of imr 4831.

 

[Nimrod1203]

Awesome information. I've shot a .270 for my whole hunting life, and have never had a .25-06. I'm excited to push it to about 600 yards. the bullets seemed to be better suited for up to 600 yards then the .270...What kinda performance can i expect?
Nimrod

 

[Loner]

The bearing surface doesn't increase that much as the bore diameter is smaller so the
circumference is smaller. It has more to do with being able to burn the powder quicker
and have a larger bullet diameter to push against. Remember the pressures allowable
are around 63,000 lbs per square inch. .125x.125 x 3.1416 = .0490875 .154x.154 x 3.1416 = .07451 . That is 66% more surface area if I didn't screw up.
the 25 06 will yield up to 3092 lbs pushing on the bullet and the .308 caliber up to
4694 lbs assuming a 63000 lb max pressure. So the more powder you want to burn
behind a smaller bullet the longer the barrel you need to utilize it, as the powder will need to burn slower to keep pressures down,
everything else being equal. Very simplistic but about as far as my math skills allow.