Most gun barrels are either 4140 or 416ss, both work great. For internal parts like firing pins, sears, triggers and such I like using 4142 pre hardened steel which is 28-32 RC. I think that a lot depends on where your shop buys it's tool steel from. Some use the 4142 pre hardened and others will buy Brake die which if I remember right is 4140 pre hard. Similar alloys are out there and all will work, the 4000 series are all good, 4140, 4142, 4340 etc.

Another great metal that we use a lot is 17-4PH stainless. It's possible to heat treat 17-4 in most shops, it needs to be brought up to 900-1000 degrees for an hour. I've taken a hot plate that was used for silver soldering parts and built a small oven out of fire bricks on top.

When I use aluminum I like to use either 2024 or 7075, they're both harder and more wear resistant than common 6061 alloy.

Here is a link to an interesting article that a guy wrote about making a trapper carbine rifle. About halfway through he explains the process for nitre bluing. I use it myself and it works awesome. It's as simple as melting "stump remover" crystals in a loaf pan and dipping the parts that you want blued. After the parts achieve the desired color you just pull them out and plunge them in an oil bath, just like when heat treating O-1.

http://www.beartoothbullets.com/tech...h_notes.htm/58

Good luck, making gun parts is fun. Here's a link to Knight Armament company. I used to work there in the R&D department so I've built plenty of guns and accessories.

http://www.knightarmco.com/

Check out their PDW, I was there for the development of this rifle and I have machined every part in this gun.

http://www.knightarmco.com/pdw.html