I found another solution to the Gunsmith part 4 that requires more flea and fewer traders. It removes the SF3P flash hider which is almost always sold out, the Mini Monster which is at an insane price right now, and the 260mm barrel which is also extremely expensive. The only bottleneck is still the 60 rounder. I happened to find a guy who got dropped by a scav with this mag so I suppose I am lucky. To try and get it you could try and hunt some raiders or even take on the rouges.
Keep in mind I'm no great gunsmith so please try some stuff out as I also picked a few parts just because I had them. If you have any suggestions to make this build cheaper