Features I would look for, considering your applications-
-16" barrel. Longer would be awkward for house defense and shorter requires a pinned muzzle device which will make it difficult to change the handguards. This last is important because as a beginner with the AR, you won't know what will work best for you until you shoot it for awhile.
-Standard handguards. They are the cheapest and easiest to replace. Unless you know what free float tube you want, start with standard or MOE handguards. If you decide to start with free float tubes, the heaviest and usually most expensive are the quad rails. The Troy Extreme has a rail only along the top, is light, made from aluminum and reasonably priced. Other rails can be added as needed. Carbon fiber handguards are also light, range from reasonable to expensive and some have rails. I chose a carbon fiber rail because they remain cool.
-Avoid the heavy barrel configuration. The heavy barrel does deal with heat better, but it's slower and the extra weight is less pleasant to lug around. I would also avoid the M4 & Government profiles. Both are lighter than the heavy barrel and will serve you well, but the weight is distributed poorly, especially the Gov that is thicker ahead of the front sight base. The notch of the M4 barrel isn't needed and does nothing. A pencil barrel profile makes the most sense. I would prefer a "Sporter" profile, but I haven't seen one for an AR anywhere.
-Look for a 1/8 or 1/7 twist. A 1/9 works well with the shorter bullets (usually of the 55 gr weight & lighter weight class) but the 1/7 & 1/8 will work better with the longer bullets (usually of the 69 gr & heavier weight class) and will still work with the shorter bullets. However, every barrel is a law unto itself. It's possible a barrel with a tighter twist will not work with shorter bullets and one with a looser twist will work fine with the longer bullets. But chances favor the tight twist barrels working with short bullets over loose twists working with long ones. No, you cannot "over stabilize" a bullet. It's either stable or it's not.
-No Rock River two stage triggers. Sorry guys, but the track record of the RR 2 stage is spotty. Some guys are having no trouble at all after thousands of rounds, but others report problems after as few as a thousand. Not just heavy use carbine class guys, but service rifle shooters as well. No AR trigger jobs. Standard AR triggers are case hardened and evidence shows the case hardening varies in depth. Some triggers have enough to give good service life after being smoothed but too many wear out quick because they were stoned too much.
-In spec BCG with properly staked gas key. While it costs money to pressure test and perform a mag particle inspection of a bolt, it assures good quality control is in place. Personally, I don't think this process is necessary for a company to produce a good BCG but it weeds out the lower quality manufacturers (for example, Knights Armament doesn't HPT/MPT their bolts and none ever fail). As cheap insurance, it's best to stick with bolts that have been HPT/MPT until learning more about what bolts & carriers are problematic and which can be relied on to give good service. Proper gas key staking is a must.
-Look for a milspec receiver extension. They are made from 7075 alloy and it's easier to find stocks to fit. Commercial RE's are made from a softer 6061 and are of a different diameter and save you no money.
Things not needed
-Dust covers & forward assists. IT's ok to choose a rifle with either or both, but they are not needed.
-Chrome lined barrel. If choosing a carbine strictly for house defense and carbine classes, a chrome lined barrel will extend barrel life at the expense of a little bit of accuracy. Still, it will take several thousand rounds to wear out a non-lined barrel. A barrel coated with a nitro carborizing process is also a good choice. The barrel of the S&W Sport is nitro carborized.
-Fancy coatings on the bolt carrier. While cycling feels smoother and they are supposed to be easier to clean, the coatings don't add anything to the function of the rifle.
-Fancy buttstocks- Save money and weight by getting a good, basic stock. The Magpul MOE is a good base line.
-Lots of rail estate. A well equipped HD carbine should have a close range optic (like the Aimpoint Micro) a good flashlight and a sling. You don't need much rail estate to mount those items.
-Hammer forged barrels. While they are good barrels, you do not need to spend extra money just to get one. However, there is no reason to not get one either
)
