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 : M4) using ctrl + F 
6. drag the M4 data and paste this :

$ M4				INSTANT_HIT	180.0 190.0	356	-1		5	riflebad	50	 30		0.74 -0.04  0.13    0  0	0.45 0.9 	 6  8  6	 6  8  6  99	7001
$ M4				INSTANT_HIT	185.0 190.0	356	-1		5	rifle		50	 30		0.74 -0.04  0.13    1  200	0.65 1.0 	 6  8  6	 6  8  6  99	7011
$ M4				INSTANT_HIT	190.0 190.0	356	-1		5	rifle		50	 30		0.74 -0.04  0.13    2  999	0.80 1.1 	 6  8  6	 6  8  6  99	7031


7. save or ctrl + s
