The heavy blaster rifle begins with the cumbersome 3 weapon quality. The marksman barrel attachment (p. 190) states that the weapon gains the cumbersome 2 quality. So does a heavy blaster rifle with the attachment become cumbersome 5 or stay as cumbersome 3?
I see it both ways. Cumbersome 5 makes sense because its a 2 meter long barrel and because most weapon qualities get added together with attachments. However, under the base modifiers section for the marksman barrel it states that the weapon gains the cumbersome 2 quality, where as other weapon qualities that are cumulative (such as accuracy gained by the same attachment) specifically say "gain weapon quality (x+1)."