You can do this glitch in the first round. Get over 1,000 points and buy the M14. Shoot a zombie, run to the Olympia and switch out the M14 with the M1911, and hold in the RB button and hold in X. To make sure you have it you should have the M1911, M14, and the Olympia.

To get more guns always switch out the Olympia because it's the easiest one to do the glitch with. You can do it as many times as you want, but if you pack a punch it you will lose it unless you have claymore, taken them out and hold X at the same time as you pack a punch.