In this game you can stack up to 6 guns that you can then fire all at once. To stack the guns however, you need a 'Gunstack Connector'. You will need 5 Gunstack Connectors to connect 6 guns together and unlock this achievement. You can do this the moment you find your sixth gun or anytime afterwards, provided you have at least 6 guns and 5 Gunstack Connectors.
To create gun stacks, you can either press when prompted while engaged with a merchant, or press when you come across one of the many bright glowing orbs that allow you to quick-save the game. On this menu, you grab the gun you want and move it across to the stack that you want to put it in.
Create a stack of 6 guns and the achievement is yours