.308 parent case velocity question

[cletus t.s.]

my question that i hope to get schooled on is regarding velocity on a .308 based cartidge. the three im asking about are .308, 7-08, and .260. first of all, am i correct in saying that the case dimensions of these cartridges are the same except for the neck diameter?
i'm fairly sure that is truth. why then does a .308 shoot a bullet of lets say 140 grains faster (according to max load data for the reloading books i own) than a 7-08 shooting a 140 grain bullet which shoots a 140 grain bullet faster than a .260? another way of asking is why can the .308 launch a projectile of greater weight faster or equal to the other two cases shooting a lighter projectile?
is it barrel twist? or does it have to due with trying to cram an equal powder charge down a smaller throat and barrel?
i'm all ears guys. thanks in advance for your time.

 

[RyeDaddy]

Re: .308 parent case velocity question

It has to do with length of the projectiles in question. If diameter is smaller, then the bullet will be longer for equal grain weight. The longer the bullet, the less case capacity you have, if you're seating them all to the same overall length. Also, the force of the expanding gas has a larger surface area to act upon in bullets with larger diameter.

I don't have any 140 grain 308 bullets to measure, but 155 grain AMAX's are 1.21" long, and 140 grain 6.5mm Berger VLD's are 1.405" long.

There's probably more to it that I don't understand, maybe someone else will come along with more infor.

 

[former naval person]

Re: .308 parent case velocity question

Area of the base as ryedaddy says. Same weight, greater area, same pressures, equals more velocity

 

[wrongside]

Re: .308 parent case velocity question

I just bought a 308 bolt gun so I don't have the info on 308 yet! I shoot a 260 Remington and load VV550 powder to push Lapua 139 Secnar. My load in Rem. brass runs about 2830fps and in Lapua brass run about 2900fps. When I work up some 308 loads I will pas them through the crono.