Hi i’m currently trying to model a thompson but im stuck on a certain section im not too sure how i would create it without totally messing up the normals and getting errors. (circled in red in the first photo)
You may need to use some triangles to split the flat part of the metal into the curved.
no trists are nessasary. use quads, sub surface modifier, and shift R to add loopcuts to get sharper edges. use a mirror modifier, and maybe do one part at the time then join them. import the referance image to blender as a background image.
Okay thanks, Triangles aren’t an issue as this is a game model and as far as im aware tri’s are good for game models. The entire section is mirrored already but i dont think its the same on the other side anyway but i’ll fix that after the fact.
The problem with adding loopcuts is that wont i add many unncessary polys around parts of the model that i dont need?
I would advise only using quads,even if it is a game model.The reason for this is to get your edgeflow right otherwise you might get artifacts even when converted to tris in another app.
Looking at the pic,you are already using too much geometry for the existing parts.
well… if you are not planning to add any modifiers or anything special then feel free to use some trists if its a game model, since they have to be low poly.
Thats the thing thats confusing me i dont understand how to get a good edgeflow when you have random bits that need to be chopped off. The grip on the right is subdivided which is why it looks like a lot of polys and the bit on the left is a little quick test which wont be used anyway.
Okay cool, i am trying to just use quads and i think ive only got the odd tri in a the moment.
So should i add edge loops around to be able to get that shape cut out or should i use the knife tool? Although i’ve found with the knife tool in the past that i get artifacts.
I’ll keep messing around and experimenting with it anyway. Thanks! 
So here is good view of slot where shell ejects. Just what kind of modeling accuracy are you looking for:
I guess I am confused. Is that divot part of the weapon for shell ejection? Or does the shell eject where the bolt rocks back and forth? At first sight it looks like the ejection port but at the same time, up close it is hard to tell.
1st watch this:
Then read through this thread:
http://www.polycount.com/forum/showthread.php?t=56014&page=214
Then to finish off, have a little look through here:
http://wiki.polycount.com/wiki/Subdivision_Surface_Modeling
Now you’ll have the knowledge to do more than just that.