I'm trying to model an M16, but it's not going so well when it comes to the magazine of the weapon.
Next to the magazine is a mesh. I want to cut holes in the mag with that mesh. But the magazine just goes invisible or connects with the mesh when trying to execute the Boolean Modifier. I have tested to invert the normals and some other stuff, but it isn't working.
As zeffii stated - you need to remove doubles on the carving object (Cube.003). There were 16 double vertices on there. If a boolean doesn't work it's probably because something in your geometry isn't making sense (even if it looks and renders fine).
Last time your issue was caused by inverted normals. This time it was caused by doubled-up verts.
I will add a tip about removing doubles to the answer I posted to your other boolean question.
Here's a solved .blend - although it will be more helpful for you to go through the steps of removing doubles yourself than to see the finished result.
Most of the common problems people experience with the Boolean modifier are generally down to only 2 or 3 things: Normals, Doubles (or overlapping geometry), Non-Manifold geometry.
Faces have a direction, it may seem like a weird idea but it's necessary in computer graphics to give a polygon (different word for face) a direction.
The fact that faces have a direction is included in the word "face", for English speakers: "what direction is it facing? it's facing that direction". Blender will give the new faces you create a direction that corresponds to the faces around it, it's up to you as a modeler to be aware of this.
The direction of the Normals determines how the objects combine when using a Boolean Modifier:
On the first image the cube and the torus have their normals facing out:
On the second image the Torus has the normals facing in:
On the third image both objects have their normals facing in:
What are Difference, Union, and Intersect supposed to produce?
Quote from the manual:
- Difference
The modified mesh is subtracted from the target mesh.
If the target Mesh has inverted normals, Blender will Intersect the modified mesh.
If the modified Mesh has inverted normals, Blender will add both meshes (Union).
If both Meshes use inverted normals, Blender will Intersect the target Mesh.
- Union
The target mesh is added to the modified mesh.
If the target Mesh has inverted normals, Blender will Intersect the target Mesh.
If the modified Mesh has inverted normals, Blender will subtract the target Mesh.
If both Meshes use inverted normals, Blender will Intersect the modified Mesh.
- Intersect
The target mesh is subtracted from the modified mesh.
If the target Mesh has inverted normals, Blender will subtract the target Mesh.
If the modified Mesh has inverted normals, Blender will intersect the target Mesh.
If both Meshes use inverted normals, Blender will add both meshes (Union).
Fixes for problems caused by normals in boolean operations are generally easy: Faces of both objects should all be outward facing with no exceptions.
To recalculate normals enter Edit Mode (Tab).
Select all faces and press CtrlN To have the normals face out
Or CtrlShiftN to have normals face inside the mesh.
To flip a single face, select it and use Mesh>Normals>Flip Normals.
You can toggle the display of normals in the Properties Panel (right panel N): 
Doubles are multiple vertices which share the same 3d location. These are problematic for the boolean modifier if they occur in one object, because they're a sign of degenerate geometry (zero area faces, hidden duplicated edges) or simple geometry which isn't fully connected as you might be led to believe by how it looks.
To Remove doubles
( insert images of example tell-tale signs of doubles )
this is a known bug in Carve upstream (a library we're using for boolean operations)
Boolean operations will fail when using Non-manifold objects.
For all intents are purposes manifold means 'is it air-tight', meaning it has no holes. Examples of non-manifold objects could be objects like planes with no thickness, or objects that are self-intersecting or that have Zero-area faces or have Open volumes.
For some operations it helps if you use the Outliner to hide the object you are doing the boolean operation with from view/render so you can see clearly the effect of the operation before applying the modifier (apply at the last possible moment!)
Using Curve objects instead of Mesh objects
Blender doesn't let you use a curve objects for boolean operations. That inlcudes Text objects, as they are considered curves as well. Those objects would need to converted to mesh by pressing ⎇ AltC.
w doesn't do this in edit mode. Maybe it has changed in newer Blender versions?
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Commented
Feb 19, 2021 at 2:57
In Blender 2.8, if boolean operations fail because of double geometry Removes Doubles is now found in the menu, Vertex > Merge Vertices > By distance.
Mesh > Clean Up > Merge By Distance
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In Blender 2.90.0 I added two cylinders, on smaller in X and Y but bigger in Z so to cut a hole in the outer cylinder. After applying the Boolean Modifier I wasn't seeing the result of the Difference option.
Only after I hid the inner cylinder from the View Port in the Data Outliner I was able to see the result.
In blender 2.91 I also got some troubles (my modifier was just removed). I ended up just clicking "self", and now it works:
For Blender 2.92 changing the solver from Exact to Fast fixed it.
In my case I was also using a Solidify modifier, which needed Only Rim to be enabled.
I had a large tattered plane (there were lots of hole in it and some non-connected parts). What worked for me was separating out the non connected parts and then cutting the remaining large piece in two (IOW, simplifying it). Now the boolean modifier works.
