RB Models blast bags

Perhaps, you should try to color tissue paper with acrylic paint and water, then dip into a solution of white glue, affix around the guns to let dry while shaping the paper while still wet.

Happy modeling      Crackers 

Are you sure they were RB barrels? In my experience, RB barrels are pretty minimalist, supplied in a tiny ziploc bag and without instructions. (Assuming 1/350 scale here, which should be p/n 350L13)  

My apologies to RB Models. I inspected the kit box again and found out it is Master Models!