Salutations!
Nice work brsseb!
Regarding the scale there are two methods I use to avoid problems:
In his tut brsseb shows us how to make a Stalker rifle. From the page he pointed us, we've learnt that the rifle is 1.08m long (42 3/4").
With this information in mind, my first step is to create a plane in front view with the X-size matching the lengh of the gun. (Create>Plane... Size X:1.08)
then load the bg texture, correct its aspect. Move two points of the plane to match height of the texture. Apply texture to the plane.
By doing this I create a modeling base "in scale". so every point created will be where it should. I can even save my file to continue later without the need to reload and reposition the texture.
and for the second method:
Let's say I didn't use the base plane and have a model in an unknown scale, as that one in the tut.
In front view I select the two most extreme points: one in the barrel tip and other in the the stock. Pressing Shift+E, a window with vertex properties will pop up. There I can see the X coordinate for the two points. Let's say -5.140 and 3.360. It means that this unknown scale rifle is 8.5m long (3.360-(-5.140)).
From the specs, I've learned that it should be 1.08m. Dividing 1.08m by 8.5m I find 0.127, my scale factor.
Once I have the factor, all I have to do is to select the entire model and scale its points 3D using the factor. (Points>Transform3D>Scale... using the factor 0.127)
After that the object will be in the right size.
good luck
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Pako Ary