Beta Weapons are weapons that were removed from a GTA game during its development. It also includes some weapons that didn't make it to the final game and weapons have their models changed, and some still exist in the game files. Many made well during development and it is unknown why they were removed. However, most of them can be obtained through cheat device and trainers.
A sniper rifle resembling an SR-25/Mk 12 can be seen wielded by both Michael and Trevor during some artworks, also in the newer update files the cut SR-25/MK-12 Hybrid HUD and texture files could be found. It is named Assault Sniper in-game.
In the trailer, Trevor can be seen holding an assault rifle with a laser attachment, the assault rifle made it in the final game, but the laser didn't, possibly due to the fact that it is useless and is not required since the player can use the HUD instead.
Some weapons had an older more realistic wood-cast model.
An M9 Pistol was meant to appear but was probably replaced by the ongoing Taurus PT92.
One of the pistols (not confirmed which one) had the same shooting noise as the pistol from GTA IV, as Lamar is seen shooting one out of a car in Franklin's trailer.
The Rocket Launcher originally had an optional grip attachment, as seen in the official BradyGames Guide. It was taken out in the final game.
An USAS 12 was meant to be in the game, it was probably replaced by the Assault Shotgun.
The Sawed-Off Shotgun had wooden grips.
A lot of the weapons in the Beta looked alot like their Real Life Counterparts (i.e the Bullpup Shotgun, resembled the KSG, alot more clearly, and the Combat MG looked alot more like the FN Mk 48) This can be seen in the hud icons for some of the weapons.