Let me correct myself, there are no machine gun coffins in “The Mercenary” or “Companeros” but both films contain scenes where Nero is running around like a mad man with a machine gun. While there may not be a widespread use of coffins containing machine guns, it has happened enough for me think its pretty obvious as viewer as to what the coffin contains.
The use of machine guns, coffin or not, is certainly prevalent and I find it to be a little overdone. I don’t mind seeing Django or The Swede wasting hundreds of baddies with a machine gun but it seems like this happens in almost every film I watch. It could just be a Corbucci thing though, I’m not sure. I probably shouldn’t be making generalizations of the genre as I’ve only seen about 9 or 10 non-Leone spaghettis and the majority of them were done by the other Sergio.