Why didnt you model the gun with more lose parts thinking mostly of seperating the barrel section and sights
and why dont you like to use crease? is there some downside to this or just preference?
Which parts you make loose is largely personal preference or up to the concept that you're working from, the sights could have gone either way.
I used edge loops instead of creases because it looks much better in almost all cases, though this might change with the OpenSubdiv crease options that are eventually coming to 2.8.
Hope that helps, happy modeling!