I was wondering if anybody knows, what speeds would be attainable with a 6.5x223, loaded to 2.800 OAL? 120 grain bullets

Most data I find is where its loaded to 2.3 OAL to fit in AR magazine.

But, what about seating the bullet out as far as possible.

This would be in a short barrel.. 16-20"

Would it help to check out the stats on the 7mm TCU/7mm-223 cartridge. I have one and from the 21 inch barrel of my T/C contender carbine 31 grains of 2230 powder pushes a 140 grain lubralox(sp) black coated winchester ballistic-tip out the muzzle at 2,365 ft./sec. I was amazed that little case could do that. It has a sharp shoulder and is probably blown out a bit like an improved case. You may have the 6.5mm TCU.