Mostly what's already been said, depending on what you want to do.
Depending on the upper / extension fit: if it slides in easily, you can loctite it in or not. There is enough evidence to say that loctiting it improves absolute accuracy. Best of both worlds is a thermo fit & I prefer that. BCM uppers require thermo fit, other may require it to various degrees.
Some free floated rails come with a specified torque for the barrel nut; SLR specs 35, WOA specs 50,don't remember what BCM specs but I think it was 45 or 50. If nothing is spec'd with your rail, I'd use 40 if it's aluminum & 50 if it's steel..............be sure to use a good lube like AeroShell 33.............I use a high temp product from Loctite. And be sure to lube the face inside the nut that tightens against the barrel extension face.
I like adjustable gas blocks, but likely you really won't need it on a rifle gas barrel...............I have them with gas ports from around .088" to .103". The .088 runs perfectly with a fixed GB & an H buffer with a Spring White (standard) spring woth .223 ammo & 5.56 ammo.
On another barrel with.103" gas port, with a carbine buffer & a slightly heavier spring, the AGB is pretty wide open to reliably lock the bolt open............so you really never know until you shoot it.
If your gas port is less than .095", it's almost a certainty that you can get by just fine w/o the AGB; on the other hand, it will work just fine if you put it on & end up running it wide open.
Good Luck.............buy as many dedicated tools as you can afford as life is much easier that way. Some kind of upper clamping block & a decent vise are a must for installing barrels.
MM
