Here's one in the factory showing how far the barrel sticks out in front of the cradle
http://www.iwmcollections.org.uk/media/images/Photographs/Image/D_011393.jpgX
Remove the X to view
Airfix's barrel may be a little short, but not much IMHO.
As for colours, SCC2 brown is possible as is SCC15 Olive Drab, with old equipment not getting a repaint just for the sake of uniformity.
David