Modeling a product database and dealing with inheritance
You might like to look here - they give a lot of sample data models. 

There are two in particular which might be of use to you. Here is a BOM (Bill Of Materials) which looks as if it might be a good fit for your business (manufacturing machine guns). Take a look at the "Manufacturing" section (no. 13).

You appear to have a table for all your different components Barrel, Buttstock, Sight and Scope amongst others. I'm not so sure that they're all necessary? But, as Einstein said, things should be made as simple as required, but no simpler.

What's your RDBMS? It's always good on this site when asking a question to provide your database server.

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