You all probably know the 3 weapon glitch, but lots of people do not know how to get the 3rd gun upgraded without losing a gun.

So the 1st gun you upgrade normally (or you can do the glitch but I prefer to do this on the second one).

Then to do the glitch, it doesn't matter whether or not you've already upgraded a gun, you buy bouncing betties then put your gun into the pack a punch then run and get either a kar98k or the gewher.

Sprint back and before you collect the gun press the bouncing betties button and as soon as you've pressed it grab your gun (if it didnt work you collected your gun to early or to late).

Now that you've got 3 guns buy new guns or whatever as you normally would, but to upgrade your 3rd gun you have to put the gun in the machine, then press the bouncing betty button and again quickly grab your new gun (it is easy but can require practice).