WHOA! That first image is awesome! Nice depth of field...looks like that thing's about to unload a round into my computer room!
Yeah, I think you just have to figure out the camera settings to match whatever kind of light source you are using. I just went through something similar and it takes a bit to get it right. Once it's locked in you should be good though. Looks like you have a good camera since the images are really sharp and have good detail and focus.
As for the "blast bag" I think Christopher was referring to the canvas mantlet cover...wondering if that's molded on or if you sculpted it out of putty or some such.
This is looking like a really sharp build here. If you can get your color images figured out, this will showcase beautifully!
Keep at it.