The way texturing. is done correct is using a UV-Map. I assume you have basic Blender knowledge.
Anyways, I'm making a tutorial. Lemme get some snapshots...
[EDIT]
TUTORIALZ!
Okay, I just edited the blender cube to resemble something that looks like a gun.
Now, I switch to edit mode.
I hit hotkey Z to enter wireframe mode,
Then hit hotkey U (Or hit menu Mesh > UV Unrwap) to bring up the UV menu.
Click Unwrap, then open Blender's UV/Image Editor.
Now, you can use functions like scale, but to move the selection, you must hit G instead of the regular moving.
Have fun mapping, check on youtube for more tutorials.