you can copy patented designs as long as you don't sell them. In actuality the patent may be up on the barret, you run into COPYRIGHT issues if your gun bears the likeness of barret's design and you sell it. So the patent issue doesn't pertain. You can also sell parts to make your not a barret look like a barret. I would also suggest that you make your production sample look the best as possible because you always want to show your best work to potential customers. Knock the burrs off, taking emery to all the flat surfaces, or if you have a lap table use it. Paint is not a big deal but surface finish is. Do you think that you will have a working prototype by the end of the month?