The AR is a bit more complex than is needed for this tutorial so I skipped a few steps that I did to prepare the model.
In the process of creating this reversed negative of the part, we will be creating several solid bodies and removing all but the one we want to keep. This AR lower receiver is a bit complicated for this as for a negative used as a holding jig we do not need any internal features.
I closed the original file up by using a lot of simple sketched using convert entities and extrude to surface. You can look at the attached file if you want more detail. The part does not need to be solid but should be water tight to make things more easy.
Here is the simplified AR lower.
on the front plane I made a sketch of a rectangle that surrounds the part. choose a sketch plane that is where you would like the part to sit in the negative and adjust to your needs.
On this part, if I need to machine out a negative I want it to be the more simple of the two sides so that is how I decided the direction to extrude. use the sketch from the previous step and do an extruded boss feature and make sure you go past the part. this will be the negative.
***Be sure to uncheck merge results!!!
Now you should be able to see that you have 2 solid bodies in your part.
Open that folder and you will see your two bodies. select them both and right click and there will be a button for the combine feature.
(I am not sure where this is in the ribbon but I think you can add it if you don't like right clicking.)
Depending on how you selected these bodies is what will be in the box. You want to make sure you check "Subtract" and then delete the bodies shown in the main bodies or bodies to combine boxes.
highlight the main bodies box and then go to your part and select the bodies you want to keep. in this case it is our extrude. and then in bodies to combine, select the bodies you want to remove. (our AR)
This deleted everything were the two bodies was overlapping. Because I had a lot of hollow areas still in the AR we are left with a number of bodies. You are given the choice of what bodies you want to keep. when you place the check in the top body you will see our part turns blue. Only the bodies we check will remain.
and this is what we are left with.