Gas length on ALL AR's classed as rifles is 5-7" from the MUZZLE. When you shorten the barrel, you shorten the distance from the chamber to port - distance from port to muzzle stays the same if optimum reliability is needed.
What confuses the situation is BATF legal M4 clones with carbine gas and 16" barrels. It was originally sold that way to placate the masses. As time went on, the makers had problems with the excessive port to muzzle length battering the action. MILSPEC guns aren't that long, they are 14.5", and it does make a difference. Enough the makers invented midlength to cut down on Customer Service returns.
The real question that needs to be answered first, before which gun, is What distances will the gun be shot, at what targets? A long range paper target gun won't be the optimum short range live target gun. It may not even be best to use 5.56, as all the hunters going alternate calibers will attest. Some can't use 5.56 hunting simply because it's not legal in their jurisdiction to hunt with one.
For general plinking, 5.56 is inexpensive, and works on smaller live game. 16" for 300 yards and less, 20" for 500 yards and less. Milspec barrels are fine, the new standard on the M4 is NO M203 cuts on the barrel. There's a lot of them still out there, and I seriously doubt they will keep being made, it's a weak point when the gun overheats. Barrel profile is no guarantee of accuracy. The HBAR is a pound heavier, but most don't report it better than 1MOA. Stainless precision barrels are generally accurate - for long range shooting or varminting. On deer and hogs, a 16" carbon barrel does fine. Goes to the question, what range, what target?
If you want more lethality or range, other calibers are available, and none are cheap. They are all commercial priced cartridges just like .30-30 or .30-06. If shooting hundreds of rounds a year and not reloading is part of your shooting style, don't get them. Stick with cheap military surplus or reject ammo.
From there, the A3 flattop is the best upper, you can mount nearly any optic to the rail. Furniture is base lined by the fixed stock, A1 for a slightly shorter length of pull, A2 for longer. The adjustable stock is next, for extra cost, and most people set it on one length, but claim they "need" the collapsible feature. Apparently they have small trunks in their car.
It's up to you to decide if a $200-350 free float quad rail is necessary. A light can be mounted to the AR without it, and the money saved will buy 1000 rounds of PMC or a turret press reloading kit. What the free float cannot do is make the barrel any more accurate that your skill and it's natural group make it. Most commercial M4's run 1-2 MOA, about a 2" group at 100 yards. That's military specification shooting issue ammo. It doesn't suddenly start shooting 1/2MOA just because a free float is on it - a stainless precision barrel is almost mandatory first. Don't get the cart before the horse.
Triggers are a huge fad now, with fanboys dropping $250 for target triggers with 2 pound pulls. Goes back to what range, what target? On a stainless precision rifle, sure. A field gun for deer, the stock GI does fine, just add an adjustable take up screw and save $200. It gives most of what the adjustable target trigger does, have a short reset and short travel. Since the sear doesn't slide over as long a distance, most of the grittiness will be eliminated, just like the common bolt action hunting rifle on the rack.
Saved money on the freefloat and trigger gives you $400 to buy the red dot or scope you want. An S&W with Aimpoint Patrol could be had for about $1000 - for example. No problem with DD as a choice.
Ask What range, What target? and narrow down what you will want to the gun to do well most of the time you shoot it, and it won't be compromised by options that work at cross purposes. That avoids the M4 with bipod and target scope syndrome that gets posted up on so many forums, a cross breed that can't excel at anything and frustrates the owner.
) whereas the GOVT. profile barrel is smooth the whole length of the barrel. It's pretty much just a preference issue there.
