Grab the new Rifle weapon model

alongside

SMG and Rifle share similar properties

Right-click on  SMG.tscn in FileSystem

Select Duplicate and rename to Rifle

Then double-click the Rifle.tscn file to open the scene

In the Rifle scene

Rename the root node to Rifle

Select the SMG model node

In the Inspector

Click the tools icon 

next to Filter Properties

Choose Copy Properties

Now drag and drop the Rifle model

Into the scene

Click its tool icon

Choose Paste Properties

Then delete the SMG model node

Select the Rifle root node

In the Inspector

lower the fire rate

Assign the Rifle Mesh

Increase the recoil rate

Increase the damage

Ensure Automatic is off

Open the hit_scan_weapon script

In the process function

add a branch for semi-automatic firing

This is the Else branch