Basically it comes down to this. Yes, you can make gun stocks on a CNC. You can do it with only 3 axes or you can do it with a 4 axis machine. A tool changer that is capable of compensating for all bit offsets and getting everything just right will make the job a hell of lot easier.
The quality of the gun stock is going to be in direct relation to the quality of the the 3D model you are generating G-code from. If you are doing custom fit gun stocks as was mentioned a few post ago, then each custom fit stock is going to have to be digitized and reprogrammed.
Known factors such as barrel and trigger housing can be imported into the 3D model and used over and over again once they are established and proofed.
All tools that are used for cutting operations will have to be carefully monitored for bitware and resharpened bits will have to be taken into consideration when you cut pre-existing g-code.
There are about 100 other things that come into play before you cut your first gun stock on a CNC machine. Too many to mention.
Can it be done? Most definately. If you are making the same stock for the same model gun on a production run, CNC is the way to go.
If you are thinking about opening up a custom business where you make individual stocks to suit a clients gun, look at a duplicator like one from Gemini in the previous post.
CNC is not a magic do all at the push of a button. The human element is missing. That element has to be added into the program by a person that is familiar with CNC and the materials they cut. For example, I cut lots of custom furniture parts in various species of wood. I would be totally lost in a machine shop because metal cuts at different speeds then wood and different cutters are used.
If you are familiar with making guns stocks the old fashioned way, then a duplicator is for you. If you want to make hundreds of the same stock in a production run, then by all means CNC is the only way to go.
In my humble opinion, Conrad Fiore hit the nail on the head with his post.
Just my 2 cents...