No, I know you can't.
But damnit, you should.
The m4, the l85 and the g36c in the game all have picatinny rails.
Wich are, say it with me, a standardized method of mounting a variety of sights.
That means, you should be able to just slide the sight of the m4 or the l85 and swap it for the pos iron sights on the g36c.
for more details:
http://en.wikipedia.org/wiki/Picatinny_rail
But damnit, you should.
The m4, the l85 and the g36c in the game all have picatinny rails.
Wich are, say it with me, a standardized method of mounting a variety of sights.
That means, you should be able to just slide the sight of the m4 or the l85 and swap it for the pos iron sights on the g36c.
for more details:
http://en.wikipedia.org/wiki/Picatinny_rail