few suggestions based solely on personal preference.
the emblem on the side should be a color form the army scheme. the reason is you can them paint the chamber relase chainmail and well as the inside of the chainmail and give them a bit more depth.
since you aren\'t going for NMM and i dont even know how to do it anyway...
take some mithril silver or a brighter metal color and highlight the areas of the clip that would not likely have dust collect in the midts of battle.
even marines aren\'t going to stop firing to field strip and clean their weapons.
do a thin chaos black wash over the metal areas to put some grime on them and darkne the recesses.
don\'t be afraid to add some damage to the gun or some imperfection to the casing. an explosive round not directly hitting the marine may still cast debris that could put pits or chips into the finish of a bolter.
i would think power armor is strong than a bolter is so if shrapnel could damage it then why can\'t it scuff up a bolter?
thinking about it i don\'t know what color for the emblem on the casing since they are for ultramariens that would look good. unless you wanted to detail the wings. the ridges a brown or gold color, and the flat parts of them whiteish or silver to give them character.
or just match them to the chest eagles.
i have tried a bolter where i basecoated it and dropped it into my sand flock and shoock it around. this polished and smoothed it a bit so the metals went on a little better looking. these photos may seem splothcy because how big they are and may not need that in real scale.
the protruding portion under the barrel itself i prefer to paint black. just something about guns in general i mostly see that part as black even when other parts of shiny.
unless the entire gun is chrome or black then it matches.
in this first picture with the emblem showing there are a few \"nobs\" that oculd use either chainmail or black depending on what color the \"rivets\" should be. unless they show correctly normal sized....
these are jsut some minor things that may enhance the bolter, but not all of them are needed, jsut some little touches that may give the bland guns a bit more character than they have from the manufacturer.
of course there is really nothing wrong with them the way they are...