Chromemoly vanadium is what the barrel is made out of. there are basically two different grades available: 4140 and 4150. 4150 is part of the Mil standard and it is considered superior to 4140. *shrug* i dunno, i've used both, it doesn't really seem to matter. I honestly don't know what passes as Machinegun Steel these days. probably 4150.
Chrome Lined Barrel is exactly that. it's a Chrome lining of the bore of the barrel. The ONLY reason you'd want this is for corrosion resistance and to stick with strict adherence to the mil standards. Honestly, no one should give that much of a S**T about the technicals of MilSpec... and yet... anyway this is a SUPER old way of treating barrels and it hasn't gotten much better. basically when they drill the hole for the barrel, they drill oversize, likewise with the cutter. the reason is because the chrome lining has to build up in order to be effective. that means it's replacing the metal removed in the rifling process. The problem i have with this process is that it does not give a very even coating all the way down the barrel, and when you look at it under a microscope, it almost looks like a wavy pattern, like an onahole (do not google that at work or around children). this wreaks havoc on a bullet and will not generally give good accuracy. sometimes you get lucky... but the mil spec is like 3 or 4 MOA at any given distance... so take that for what it's worth (about 4.5 to 5 inches at 100 yards) MY preference is QPQ, also called Nitro-carburization, Salt Bath Nitride, Melonite, Life Coat, RF85, ...I think Glock calls it Tennifer?... and a whole host of other "trade names". it is nothing more than a crazy hot soak in a vat of salt nitride for a certain amount of time. it is then removed from the vat, cooled, polished and dipped again. QPQ stands for Quench Polish Quench and the process could not be more litteral. When it comes to corrosion protection, it's tops. and the best part is that it's harder than hard chrome, and is absolutely flawless down the bore. it's actually treating the metal, not building upon itself so there's no compromise for the barrel maker in keeping the bore tight. these are no doubt better barrels.
Cold Hammer forging is a process by which the rifling is created. you basically have three methods. CHF, Button, and Single point. CHF means they drill a barrel, take a shaped mandrel, stick it down the hole, and let tons and tons of hydraulic pressure pound the steel into the negative shape of the mandrel. it's cheap, it's fast, it's accurate, but holy **** does it stress the metal, and more often than not it's never de-stressed. many high end barrel makers will destress their barrels, or ensure that no stress goes into them for accuracy reasons. the second is button, and it's exactly what you might imagine. they take a carbide "cutter" that has six (or 4 or 5, depending on the pattern of rifling involved) sides to it, and drive it through the bore to create the grooves and lands. again, this stresses the metal nearest the bullet, but not as bad as CHF. still widely used. the last you will not see on cheap barrels. Single point is ARGUABLY the best way to rifle a barrel, the slowest, the most labor intensive, and oldest. it uses a single carbide cutter head to make single passes down the barrel taking off small amounts of material on every pass.
When it comes to gas length... you have to settle on a length. i'll tell you right now that the carbine length systems are design primarily for 14.5" SBR (Short Barreled Rifles, NFA stuff). anything longer than 14.5" on a carbine system will be over-gassed and will beat up your rifle a little more than normal. anything shorter may not function correctly (but this is rarely the case). Mid-length was the solution to those who wanted the shortest barrel possible without having to get a $200 tax stamp, which is 16". these also work great on 18" barrels as well. Rifle length systems are the standard for any barrel 20" or longer, and there exist some special extended rifle systems out there for the truly insane who want a 24 or 26" AR15. something about strokes and folks. anyway if you're dead set on a 16" gun, and you want the simplest of installations, a Mid-length gas system is your ticket. there's hardly any buffer weight tuning involved, you can use just about anything and get away with it, and it'll run and shoot great. a carbine will give you a harsh recoil and like i said will beat up your rifle a little more than normal, but it will be ultra reliable (at lest for a while anyway). Just don't suppress it, you will hate life.
I think you're over thinking this build really. this isn't going to be your last AR15, that i can freaking guarantee. Buy a completed rifle for your first one. something simple and reliable. Colt makes a GREAT rifle and so does BCM and hell even PSA and AERO Precision won't drop **** on your doorstep. It's gonna run, it's gonna shoot at LEAST 2 MOA with just about anything, and you're going to start to wonder how things work. pretty soon you'll start answering your own questions as you get to play with the rifle and make changes and see the results. will you want to change things on the rifle as it ages? most definitely. and when you get to the point of wanting to build one for the first time, you're going to kick yourself for not doing it in the first place. Just remember, you would not be where you are today had it not been for that first rifle.
remember that and you'll be just fine.
Christ, that was long winded... i think i need a smoke!
motivational speeches are not free of charge and i will be sending you an invoice for my services.
nah just kidding. I love this ****.