You can create custom weaponry from deriving from one of the following classes:

• Gun (Gun provides common useful methods such as ShootBullet that gun like weapons would find useful, it is derived from Weapon)
• Melee (Melee has an Attack method, it is derived from Weapon)
• Weapon (Weapon provides reload and ammo system and no more, it is derived from Carriable)
• Carriable (Carriable provides nothing, its a bare bones thing that goes into your inventory and has a viewmodel)

heres an example weapon you might find helpful

using Editor;
using Sandbox;
 
namespace MyGame;
[Library("ttt_weapon_overpowered"), HammerEntity]
[Title("Overpowered Weapon"), Category("Weapons")]
public class OverpoweredWeapon : Gun, ISecondaryWeapon, IRandomWeapon
{
	public override string ViewModelPath => "weapons/rust_pistol/v_rust_pistol.vmdl";
	public override string WorldModelPath => "weapons/rust_pistol/rust_pistol.vmdl";
	public override float PrimaryAttackDelay => 0.000f;
	public override float PrimaryReloadDelay => 0.000f;
	public override int MaxPrimaryAmmo => 100000;
	public override AmmoType PrimaryAmmoType => AmmoType.Spare;
	public override float HeadshotMultiplier => 10.00f;
	public override bool Automatic => true;
	public override void PrimaryAttack()
	{
		PrimaryAmmo -= 1;
		ShootBullet(1000, 0.0f, 1000);
		PlaySound("rust_pistol.shoot");
		(Owner as AnimatedEntity)?.SetAnimParameter("b_attack", true);
		ShootEffects();
	}
	public override void ReloadPrimary()
	{
		base.ReloadPrimary();
	}
}