I have routinely made my .300's out of .308 brass with zero issues. As for rounds 4 & 5 hanging up I can't begin to imagine what precisely is the issue without dismantling the gun. I would look first at cleanliness and complete disassembly is the only route to that. Rotor spring tension would be something to consider at the same time, as well as confirming that nobody swapped out the rotor in the past with a wrong one, ditto the cartridge guide on the RH side looking down from the top.
