I assume by look you mean that you want to edit the sprite.
Step one is probably to get the weapon sprites. Each weapon has 3 kind of sprites:
Bigobs = Inventory pictures, Handobs = the sprites drawn as held by the soldier/alien for each of the 8 directions and floorob = the sprite on the battleScape floor/ground.
You can obtain the original files when you use Pckview as part of
MapView to open the game files. MapView is actually a tool to create map tiles with but contains additional tools. The interesting files can be found in the UFO/UNITS directory: BIGOB.PCK, FLOOROB.PCK and HANDOB.PCK. Using PckView you can export the sprites as PNG for further editing.
Note that when you add elements to
extraSprites Handobs will use 8 indices. That means if your gun handob with 8 directions is index 200, the next one should be 208 (200+8).
As for tools to edit the sprites with you can search around on the forum. Some are listed
here and the community has made additional tools such as the
UnitSprite Studio.
Edit: I meant Handobs when talking about the 8 indices with the 200+8 example