locking shoulder is a part of the upper receiver of an FN FAL Belgian Rifle
LOL The apocalypse isn't that bad. We have batteries and small electronics.
First - think about hiring a handyman to do this. Soldering pipe isn't hard, but learning how to do it while under your house where a leak would be hard to find and repair is not easy. The peace of mind might be worth it. Be sure to ask for an estimate on the price, both for material and for labor, before you agree to anything. If you have to or really want to do it yourself, find a book on DIY plumbing tips. You can probably find one at almost any home improvement stores, like Lowe's, Menards, Home Depot, etc. Then read the parts on making solder joints carefully. Get some pipe and fittings, and practice making joints in the garage, back yard, or someplace well ventilated and easy to work in. Under the house is NOT the place to try this. I'd recommend you try to make at least 20 good joints, and test them by putting them under water pressure to see if they are good. You'll need a propane torch, solder, flux, pipe, fittings, a pipe cleaning brush, and a tubing cutter. The last is a gizmo that cuts copper pipe easily and cleanly. It's MUCH better than using a saw. The brush is a special wire brush that scrubs the corrosion off the inside and outside of the pipe, to help make a better joint. Oh, and get ready for a case of sticker shock when you go to buy copper pipe. The price of copper has skyrocketed lately, so you'll want to be careful to buy only the sizes you need. Read the book about the different kinds and sizes and thicknesses of copper pipe, and look carefully at what you have now, so you know what to get. But you'll still be better off hiring someone to do this, I think.