How to edit weapon.dat :

1. Open your GTA SA Directory (My example E:\GTA SA Folder)
2. Open data
3. search weapon.dat or weapon
4. open using Wordpad or Notepad
5. find your weapon to edit (example : Desert Eagle) using ctrl + F 
6. drag the Desert Eagle data and paste this :

$ DESERT_EAGLE		INSTANT_HIT	45.0 90.0	348	-1		2	pythonbad	6	 70	0.41  0.03  0.12    0  0	0.75 1.0 	 6 28  6     6 28  6  40	7001
$ DESERT_EAGLE		INSTANT_HIT	50.0 95.0	348	-1		2	python		6	 140	0.41  0.03  0.12    1  200	1.0  1.2 	 6 25  6     6 25  6  40	7011
$ DESERT_EAGLE		INSTANT_HIT	55.0 95.0	348	-1		2	python		6	 140	0.41  0.03  0.12    2  999	1.25 1.5 	 6 22  6     6 22  6  40	7031


7. save or ctrl + s
