When you are playing as John Morgan, enter rage mode. John will pull out a staff, which is separate from his standard inventory. If your inventory is full at that point and another weapon pickup is nearby, you can quickly select that other weapon to replace the baton that John is carrying. That will add an additional weapon to your inventory, so that you are now carrying more than the usual maximum of 18 weapons. You can continue to expand your inventory in this manner, by activating rage mode and swapping out the baton for addition items as your inventory swells.