A. Installing:
1.Than take an IMG-Editor to open up the gta3.img and delete "m4.dff" and "m4.txd" ,than add the ones you just downloaded, after thats done REBUILD THE ARCHIVE !!!
2.If you wish to have the original Name of that new Weapon,open up the GXT of viectiy with a GXT-Editor and open the line of the m4 when it's open replace the Data with "Sig-Arms Sturmgewehr 552".

B. Credits:
The whole gun is fully made by me in work of about 5-7 hours

C.Disclaimer(Don't Mind about damage or not working,if installed right ,nothing will be damaged and everything should work fine):
Even if this Mod was done for a product by Rockstar Games, it ain't supported or made by them,
you use this mod on your own risk, so damages of your PC shouldn't be anyone other's care than your's.
Although that Mod was properly tested and shouldn't damage anything, i can't gurantee that it works on your system same well as on my one, as i don't have
the material to test it on every possible system.

Silent Viper