The 7mm int rimmed is based off the 30-30 cartride so the powder capacity should be about the same as the parent case. 7mmbr looks like it has a lot less powder capacity. Why is the 7mm int rimmed loaded so much lighter than either one? Is it a function of the guns initially created for the 7mm int rimmed? Is it a function of case dimentions other than capacity? Any body have any ideas?

It's a function of the strength of the parent case, the 30/30 which was made for lever actions. Also, the shoulder is pushed back a ways which reduces the powder capacity. I can't quote pressures off the top of my head, but the case I believe is the achilles heel.The 7 BR of course is based on the .308 case family and can be loaded to higher pressures

Actually, the strength of the case is of little consequence here. The 7mm International Rimmed was originally designed for use in single shots such as the TC Contender, and therefore pressures must be limited somewhat to minimize backthrust against the frame (though there is MUCH MORE to the equation than simple pressure when it comes to backthrust and safe loads for the Contender).

The 7mm BR is not that far behind the 7IR when it comes to capacity, but it is loaded to higher pressures -- and that's what accoutns for the difference. However, from a 14" barrel, the 7 BR will push a 120 grain bullet to 2450 fps while the 7 IR approaches the 2400 fps mark. So despite differences, the performance of the two is still quite similar.