I wasn't sure where to put this question, but some people here would have a greater knowledge of ballistics than in the reloading forum so I chose this one.

Looking at loading data and charts it appears that the 7mm-08 cannot generate teh same velicities as the .308win with the same weight bullets.
Why should this be?
Especially if you use .308brass necked down, I would like to think that you should be able to get the same velicities.
Maybe there will be a slight presure problem as the comparitively longer bullet will encounter more friction in the barrel but otherwise I can't see why you shouldn't be able to load 7mm 165gn bullets to the same speed as 165gn .308 bullets.

What gives?

Well, basically your chamber is a high pressure balloon and the bore is letting the air out. The bigger size of the 308 bore will let the gas out quicker than the 7mm bore of the 7/08. In this case the 308 load can use more/faster powder and still be held to around the same pressure levels because of the greater size of this gas outlet.

Ok, I like that answer, but usually you can get higher velocities with "overbored" cartridges, can't you?

Granted, you are also normally using lighter bullets though...

So that's it?

As referenced above it's the surface area of the bullet that get's pushed on by the gas. The larger it is the faster it goes other things being equal.

Here is a list from memory of what the cartridges on the "308" case do for muzzle energy. The 243=2000 fpe, 260 Rem=2250 fpe, 7mm-08=2400 fpe, 308W= 2700 fpe and 358 Win 2900 fpe!

In general the larger bore always wins down range as well as a sharp pointed meplat on a large bore is more important than a smaller bullet starting out slower even if it's ballistic coefficient is better.

Still waiting for Sabot to send us some samples.

Overbore cartridges usually mean a case that holds a lot more powder for a given bullet diameter then is the norm. A 7mm-08 really isn't overbore, a 7mm STW or Ultramag is. You get higher speeds by having a bigger balloon (I really like that image), or a larger case full of powder. The overbore definition really doesn't work for very similar cases in the comparison. - Dan