Using Dillon's USPSA Power Factor Calculator, the standard ball round from the old Czech VZ-52 carbine comes in at 327.5;  131 grain pill at 2500 fps.  And it fits into a AR magazine!  I think I'll try it out at our next local competition!  Using my ancient VZ-52....  If all goes well, I may just build up an upper with that cartridge.

I think wilson combat developed a 7.62x45 cartridge. You might want to czech it out.

I wouldn't even worry about power factor if you just want to shoot the rifle for fun.