This requires you to modify a file. Proceed at your own risk, and remember to make back-ups.

Go to your savefile (e.g. C:/Deer Avenger 3D/savegame). You will see a list of the games saved on your computer. Using a text editor (like Notepad), open the file you want to get all weapons for. You may see some boxes. Enter the words "ALL GUNS" (case sensitive) as the new first line. Now save the file, run Deer Hunter 3D, and load your game file.