Why .260 slower than 708?

[urbaneruralite]

I looked at ballistics charts for factory ammo for .260 and 708 140 grain ammo earlier. I noticed that the 7-08 was listed as faster and, for short range, flatter shooting trajectory. I can make some suppositions about diameters and burn rates and such, but I have no concrete idea why this should be so.


Why is it?

 

[steve smith]

For any given case capacity(the same parent case 308,223,30-06), as you increase the diameter of the projectile while leaving the mass the same you will see an increase in velocity potential. So if you had a 338-06 using 180gr bullets and a 270 with 180gr bullets, the 338-06 has more velocity potential with that weight bullet.

Look in just about any reloading manual and you will see this.

As for the reason, it has to do mostly with the cross sectional area of the bullet. The more area the expanding gasses have to push on the easier it is to push at a given pressure.

 

[britz]

THis discussion went on earlier (don't remember who exactly was stating it, but it made sense). If you have the identical burn rate, and the same case dimensions, same weight bullet but different caliber, the larger cal will always go faster out of the muzzle. Take a WSM case that is getting around 63,000 psi. The 270 WSM has far fewer square inces to have force excerted on it than the 300 WSM. Therefore the 270 will not get the net force that the 300 will if using the same weight bullet.

However, you spoke of short range balistics...

the smaller diameter bullet of two bullets of the same weight and same design and different calibers will have a higher balistic coefeicient causeing it to carry better and eventually out preform the shorter fater bullet that is going a little faster out of the muzzle.

 

[urbaneruralite]

Thanks, guys. Its always nice when the answer matches your supposition. I guess I was looking for a better characterization than just a notion.

I was also thinking that bearing surface might be a tiny factor. Can't come up with the math for that either, though.

 

[britz]

I was bored and did a little math. I thought that the bering area would go up as the cal went up. Not the case.

I took 2 barns bullets, 160g 284 and a 165 grain 308. I guessed that .666 of the bullet length for that particular brand was baring and I calculated the surface area of the two. the .284 had a berring surface of .75427" squared and the .308 had a bering surface of .7324276 because of the great affect that bullet length has to do with the surface area (Pie D times H). So the short fat bullet has the best of two out of three, loosing bigtime in BC.

This all comes out on paper, but I just looked at some loading charts comparing same weight bullets of a 325 and a 300 wsm, getting pretty much the same velocities out of same weight bullets. Go figure?