How do I move only selected meshes? I am currently modeling a type of gun known as the Scar-L. I am having trouble with this though, I extruded a piece out that will later connect to the magazine.

 

As you can see, I want the selected orange lines to scale, based on the y axis, down to reach the blue lines. I have tried this by selecting the sides faces, pressing s, then y, but it will stretch. Like shown in this picture:



I am trying to figure out how to avoid that and only scale inwards the selected meshes and not the entire thing. Any help would be appreciated! :D
If I understand correctly, Please try this:

Select your source faces:



Hit E and S (E for extrude and S for scale) and drag mouse inside to scale 

You will get some thing like this:



You also lock the scale to y if you like by hitting Y after S
Final command will be E and S and Y



Best of luck :)