Milsurp stocks are hard to make money on. People want originals, and if originals are unavailable, the milsurp in question needs to be valuable enough to make it worth while, and the sporterized versions plentiful enough to create demand.
For example the Ross. Rosses seem to be selling in the 1000+$ range, which is pretty expensive in comparison to many milsurps. There is also a large number of sporterized Rosses that can be had for 100-300$. That means if a stock can be made for somewhere in that margin, you can make a profit. That being said, nose caps are next to non-existant, so you still have to take consideration of that.
I understand where you are getting at with your thinking, I once considered it as well, but if it is for anything other than a hobby, you likely won't be able to make enough to keep the costs low enough for the market.