Since you asked for in depth Feedback i will be a bit more picky, none of those are huge issues, i think the model looks really nice, but could be better.
- there is a odd hick there, just looks weird.
- topology looks a bit odd there, also try to not have open areas like the cut on the left, there might be a situation where the light reflects through the gun in a weird way because of that
3, more geo that some is turned into tris, some into quads, id avoid any weird quads and rater turn them into tris
4. same issue here, some lines just seem to end , some are connected fine.
5. the uv seems like it got a bit of a lot of free space
6. also related to this if you have a look at some of the overwatch models, they often “cheat” by using cards for the symbols, giving that slightly more uv space so it looks better.
if you look at genji for example
Your idea looks fun although really impractical to turn on the gas, the blade would probably break if you try to hit someone as the holders don’t seem to stable and if someone uses a knife to block it they could hit the gas tank and that would destroy someones hand rater quickly (aside from the fact that gas right next to a chamber that sparks gunpowder might not be the best idea) 
and 7.
unless you need to specifically take it apart you probably can avoid parts you won’t see and make it with a few polys saved and some more texture space.
don’t get me wrong i personally love details like this, but in a game specifically you will have to focus on what the gun can do and what it can’t and try to make it as pretty as possible with as little as you can. So ignoring certain insides can often lead to that effect
