A. Installing:
1.Than take an IMG-Editor to open up the gta3.img and delete "chromegun.dff" and "chromegun.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 Chromegun ,when it's open replace the Data with "Benelli M4 Super 90 Tactical Police Shotgun."

B. Credits:
The gun was fully made by Silent Viper (bse_silentviper@silentviper.net)
Thanx to Killer.ip of GTAFORUMS.COM for providing me with a tutorial that helped me learning UVMappping
big thanx to Benelli.it for letting me download that cool pic if the M4S90 which i used as texture

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.

Note:
Don't Wonder bout why the thing has some white parts, that happened due to having about 700 faces to texture,and selecting them and even placing the UVMap right is VERY hard, and as it looked much like a kind of shiny effect, I let it in, so
I hope you understand me :)