but there are no minigun bullets it uses 1- plain bullet
so I thought by changing the number of bullet it changes the kind of bullet and remaking minigun gfx will change the graphic and when minigun will shoot plain bullets or m79 they will look like however I drew them....
That's right, that's how it should work. My guess is that maybe you modified the wrong bullet graphic or selected a wrong bullet type. Also keep in mind that Soldat uses .png files instead of .bmp if it finds them. So for example, if you have bullet.bmp and bullet.png in weapons-gfx folder - Soldat will ignore bullet.bmp and use bullet.png instead.
Also nevermind what I said about bullet trails. I think there is actually a graphic for them somewhere, only grenade and M79 trails are unchangeable I think.
I'd doublecheck all this but I'm not on PC right now, so I'll try to do this tomorrow unless you figure it out yourself by then.