How to edit weapon.dat :

1. Open your GTA SA Directory (My example C:\Program Files (x86)\Rockstar Games\GTA San Andreas, E:\GTA SA Folder or whatever your GTA SA Directory name)
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	riflebad	50	 40		0.74 -0.05  0.13     0  0		0.45 0.9 	 6 10  8	 6 10  8  99	7001
$ M4				INSTANT_HIT	150.0 190.0	356	-1		5	rifle		50	 40		0.74 -0.05  0.13     1  50		0.65 1.1 	 6  8  6	 6  8  6  99	7011
$ M4				INSTANT_HIT	190.0 200.0	356	-1		5	rifle		50	 40		0.74 -0.05  0.13     2  999		0.80 1.1 	 6  8  6	 6  8  6  99	7031

7. save or ctrl + s
