I am new to reloading and I have a dumb question..

Looking through the Barnes Reloading Manual I noticed that for a 150 grain XBT bullet for both the 308 and 7mm, the 308 flies faster and flatter.For the same type of powder they suggest loading the7mm with less powder than the 308? Why is that?

With the improved BC of the 7mm, I would have expected a performance improvement over the 308 bullet.

It really is very simple in that if two cartridges use the same case,and the same design pressure,the cartridge with the larger bore will drive the same weight of bullet at a greater velocity.Another example would be the 270win and 30-06 with 150gr bullets.The 30-06 has a larger bore so it will drive the 150gr bullet faster despite the better ballistic co-efficient of the 270 bullet.However as the range increases,the higher ballistic coefficient of the smaller diameter bullet will result in it maintaining it's velocity longer and closing the gap on the larger diameter bullet.I would als be very leary of the barnes manuals as many reloaders have foundthem to contain a fair amount of questionable data.

Whitehorn, another thing it has to do with is bearing surface. There is more of it on a 7mm 150gr bullet than a 308 150gr bullet. More surface, more pressure

the 7mm bullet has to stick into the cass farther there for less room for power

What????????

What do you mean what? If you exploded a 1 stick of dynomite in a large warehouse the overall effect on pressure of the warehouse would be much lower than if it went off in small house. Wouldn't you say?

Simply want to know WHY the 7mm bullet HAS to be seated deeper into the case than a .308 bullet would in it's case---- I've always let the bullet ogive and seating it just off the rifling lands govern seating depths. That's what is meant by "What"??

I think the basic concept here is that both calibers are usually restricted to a overall length of 2.800 by the magazine box of a short action. So when one bullet is longer and slimmer than the other, the longer bullet has to be seated deeper in the case.

Having said that, however, I believe the increased bearing surface of the longer bullet has more to do with creating pressure than seating the bullet deeper. I have built some rifles on long actions and chambered them for short action cartridges - just so they could be seated out to as long as the throat would allow. And also played around with custom long throating. The extra length does affect the pressure to velocity ratio, but its not a real big deal. Maybe 50 fps in a medium caliber.