Note: this will only work if you have not patched the game!

Once you have 2 guns, go up to any gun on the wall or, buy a gun from the box.

Press RB to throw a grenade right when you press X to get the gun.

So, you go up to a gun and press X and RB at the same time to trade in the grenade, giving you 3 weapon slots.

It will work until you have 36 slots, then you cant get any more.