Yes you're right.
For fun, I ran a 223 case through a 30caliber expander mandrel and measured the case length. It's approx 1.725" long. I plugged that case length into quickload overwriting the existing 7.62x40WT cartridge length. I changed the case length and used the shortest bullet (150 grain) seated as short as possible to maximize that magazine length. Playing around with different powders and charges, I still couldn't even get quickload to tell me that I could expect a 150 grain bullet to exceed about 2400 fps without going way over 65000 psi.
I know the case dimensions you would have after expanding the mouth on a 223 are not the same as a 762x40 stretched to 1.725, but there may not be enough additional case capacity to get me there? (just guessing, I could be wrong)
Quickload does think you can get a Speer TNT 125 grain bullet speeding along at 2601 FPS in a 22 inch barrel using 27 grains of AA1680 in my mystery cartridge. I'm really just curious about maximizing the 223 case in a std. AR-15.
I did just receive my 300OSSM upper last week. Well ahead of schedule and have not had time to get it out yet. If it runs correctly and as advertized, my biggest problem may only be keeping up with my brass?