When playing CTF go to the axis base and grab the flag. Go up to the second floor and at the far left there is a broken window. Jump onto it and jump onto the barrel. Look down and jump and let go of the flag(you must do this quickly) and you are on the roof.
In the Mace mission at the stage where you have to stop the Nazis from getting to the trenches don't listen to what the men tell you, go right and follow the path and you will find a FG42 scoped beside a crate.