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	110.0 190.0	356	-1		5	rifle		100	 30		0.74 -0.05  0.13     0  0	0.45 0.9 	 6  8  6	 6  8  6  99	A018
$ M4				INSTANT_HIT	150.0 190.0	356	-1		5	rifle		100	 30		0.74 -0.05  0.13     1  50	0.65 1.1 	 6  8  6	 6  8  6  99	A018
$ M4				INSTANT_HIT	190.0 200.0	356	-1		5	rifle		100	 30		0.74 -0.05  0.13     2  999	0.80 1.1 	 6  8  6	 6  8  6  99	A038


7. save or ctrl + s
