For your specific requirement, you would be really far better off to get some graduate engineering student to do this for you for a few crates of beer
Those web sites just give you an overview of what FEM is.
If you get an FEM package and play with it, you will probably work out how to get a mesh applied to the part, apply some pressure to the face of the bolt to represent the pressure in the chamber, and get the software to deliver a pretty colored picture of a distorted bolt showing that you maximum stress occurs somewhere in the radius of the lug or on the point of contact. You tweak the lug radius and get all the colored zones down to an acceptable stress.
The scary part is that this picture has a strong reassuring effect on people, even though it is nearly completely worthless information.
I've had a Prof at Cranfield introduce CFD as Color For Directors
A good example is a Turbine disk burst which happened in commerical aviation because the designer (who would have really known his stuff) had a slighty too coarse mesh on a radius.
He would have had a pretty picture showing that all areas of his disk had safe stress margin. Once they analysed the fractured disk and identified where the crack initiated, they tweaked the model and the mesh size locally till it explain why it happened. In the future they have a better model and better set of assumptions.
You would be better off to copy the lug design from a proven rifle using the best steel and controlled heat treatment.
I have never used Solid Works.
My experience had been with AutoCAD and then Catia.
CAD is used to design and size a part (with the modern packages adding features like building assembles and simulating motion etc).
CAM software is used to create a tool path from that part.
A post is needed to translate the tool path to a G-code file for your machine tool. The post is normal a customize plug-in to the CAM software.
G-Code is the generic name for the ASCII tool path file which will be feed to the machine. The original standard RS-274D is not strickly adhered too, beyond the basic moves. Most machine controllers have a specific flavour of G-code.
There are integrated CAD/CAM/POST systems for sale, some of which are even affordable. Dophin and Vector have good reviews but I have never used either.