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;
using TerrorTown;
 
[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();
	}
}

if you would like to make a scope similar to the one on the m24, you can use this code snippet taken from the m24

	public override void SecondaryAttack()
	{
		base.SecondaryAttack();
		if ( Game.IsClient )
		{
			if ( !Scoped )
			{
				ScopeIn();
			}
			else if ( Scoped )
			{
				ScopeOut();
			}
		}
	}
	ScopeOverlay ScopeOverlay;
	void ScopeIn()
	{
		Scoped = true;
		(Owner as Player).CameraController.SensitivityMultiplier = 25 / Game.Preferences.FieldOfView;
		(Owner as Player).CameraController.FOVMultiplier = 25 / Game.Preferences.FieldOfView;
		if ( Game.IsClient )
		{
			ViewModelEntity.EnableDrawing = false;
			ScopeOverlay = new ScopeOverlay();
			HUDRootPanel.Current.AddChild( ScopeOverlay );
			PlaySound( "sniper.scope" );
		}
	}
	void ScopeOut()
	{
		Scoped = false;
		(Owner as Player).CameraController.SensitivityMultiplier = 1;
		(Owner as Player).CameraController.FOVMultiplier = 1;
		if ( Game.IsClient )
		{
			ViewModelEntity.EnableDrawing = true;
			ScopeOverlay?.Delete();
			PlaySound( "sniper.scope" );
		}
	}
	public override void OnActiveEnd()
	{
		ScopeOverlay?.Delete();
		if ( Owner is Player player )
		{
			player.CameraController.SensitivityMultiplier = 1;
			player.CameraController.FOVMultiplier = 1;
		}
		base.OnActiveEnd();
 
	}