It's finally finished! :-)
I didn't like the silver bits I added earlier, so I sprayed them black again. Also I saw a real M1A1 and it had those parts black.
I added some scratches here and there to make it look like it was made of metal and had been used in combat. This was done by sticking the wooden end of a small paint brush into some silver paint and scratching silver onto the parts where I wanted "metal" to show through.