I’d like to hear some opinions on the M16 vs the M4, I find myself drawn to the M16 but I really don’t know why. I guess it seems more like a real rifle, though I realize the M4 is probably the better all around gun.
How big of a difference does the longer sight radius and higher velocity from the 20" barrel make? How about the smaller size/weight of the M4? Does anyone prefer the M16 over the M4? I would like to make a purchase soon and need to make a decision 
M4 all day, any day- unless shooting HP competitions.
It is more maneuverable, compact, and lightweight.
Sight radius is irrelevant- use a quality RDS, magnified w/ MRD, or variable optic.
Quality ammo makes the carbine very effective in its most likely engagement distances. Accuracy between the two are virtually identical.
I personally prefer 16 inch mid-length guns for general use.
I like 14.5 guns, but since I think the NFA hassle is a bit much for 1.5 inches of barrel, I perm-attach a FH, which many do not like.
Look…I’ve got like 6 AR’s and all with a different purpose and the last two are 16"ers (well…14.5"ers without add-ons).
The muzzle velocity loss is nominal unless you’re shooting at REAL distances for score. The sight radius is only an issue if you’re going to be ONLY using iron sights (and at that, depending on the use of the rifle, can be fixed).
My work/teaching rifle is a M4 carbine gas and a standard FSB, EOTech and BUIS on the rear. Simple. My new competition rifle (3-Gun) is a 16" mid-length gas with an ACOG or Accu-point (depending on the match). Each one does something a little different. If it’s a match with medium/long stages, I break out my 18" with rifle length gas system (and yes the 2" makes a difference over a 20"). Lastly, I have my 20" JP for wide open, LONG distance matches (like where I have to break out the 77’s and the occasional 80).
The versatility of the rifle is its beauty. So…in a short answer…buy both. Buy whatever lower and upper combo (let’s say a 20") and then buy a 16" upper.
As far as performance go, the only real difference between the two is the gas system. Not a hard fast rule, but in general and without going into port pressures and gas impulses, a carbine gas system vs. a rifle gas system is a little less reliable and more harsh on the brass. That said, this is all manageable and with some “tweaks” can be smoothed out without sacrificing “real world” usability.
Rich
I’m a little near sighted. So the Rifle sight radius makes a big difference to me. I can see targets clearer and farther for what ever reason.
That said, I almost never shoot my 20 inch gun these days.
After owning 20 plus AR’s in my life I don’t even think of buying a 20" any more. The carbine’s are nearly a definition of versatility. In the end it is your personal decision, I don’t think either way is really ‘wrong’.
The current M-4 is what the original M-16 should have been from the
drawing board.
It took over 40 years of evolution to get to where we are now.
Why look back?
The differences between the original M-16 and the current M-4
are refinements in the original product that have been mandated
by four decades of use and numerous military conflicts.
Do we still watch black and white TV, drive model T Fords,
use outdoor toilets?
It depends on the distances you plan to fire?
Out to 300yds= Not a lot of difference.
300 to 600yds= A little bit of advantage goes to the 20 inch barrel.
Beyond 600yds= Get a bigger caliber.
Sight radius, schmight radius. Lets see if you notice any difference with this setup. Not to mention the BUIS are not used that often, its mainly the M4S.
Carbines are the way to go. Rifles feel different, and admitedly, I like the way they feel, but to be honest, feeling doesn’t enter the equation. Carbines handle better, point better, maneuver better, and can even give you more options for accessorizing.
For me it’s obvious at 100 yards. If I prone out, I can guarantee a head shot at 100 yards on a rifle sight radius. I can hold 2 MOA on a hundred yard target, and in good lighting conditions get close to 1 MOA.
I can do about the same group sizes with a carbine, but at half the distance (50 yards) The carbine is probably just as accurate, but I can’t see the 4 MOA grid clearly at 100 like I can when I’m on the rifle.
When I was a young man I used an M16 (no forward assist), then the A1 and then finally the A2 as a paratrooper. As a paratrooper, I wish I had the M4 to jump with and hump thru the boonies.
As I get older that M4 gets sweeter and sweeter - after all the advice and reading the chart, I decided on the LMT and have not regreted it since:)
BUT, for shooting long range, I would choose the 20inch gov. profile A4 with optics.
It all depends on what you want to use it for. Lot of good advice posted here, but it all comes down to YOUR intended usage.
If you will be using open sights at longer distance, then the 20"er will have an advantage. With optics it’s not much of a difference. Generally the M4 will be more versatile, especially given the loger range limitations of the 5.56 cartridge.
I agree with getting a A4 for the 20 inch if your going that route. I likw to keep 1 20" AR, why I really dont know. I just like having one, and I do like shooting it at the range. But thats all I use it for. My duty gun is 16" M4gery. The 20 is just too long. Now that being said, I can shoot the 20" alot better using the irons than the 16 past 50 yards. :o
+1 well said…perfect
For an issue weapon - I’ll take an M4, but that’s because our procurement can’t get us the A5 stock for the M16’s in our inventory.
For a personal weapon - run a simple 16" Midlength unit, and an adjustable stock. If you want something beyond a bone stock, basic KISS version, get a free float handguard that extends past 9", and mount your iron sights on there - big sight radius without having a huge long musket.
What few things a 16" rifle can’t do well, 5.56 isn’t the ideal caliber for anyway.
Since it is your first/only AR I would recommend going with the M4. Not that I think there is anything wrong with a 20” rifle it’s just that I feel the M4 is more versatile do to it’s size.
My unit in the Army decided to go with A4 rifles instead of the M4 for the standard infantyman. We cleared hundreds of houses, buildings and bunkers with thos rifles. Yes the M4 was easier but its not necessarily hard with a rifle.
The M4 is a bit lighter and easier to handle when operating out of vehicles. I believe the rifle is better for long range engagements.
I strongly prefer the M4 over the M16. It is lighter, more manueverable, and I found the accuracy in my hands to be equal to the M16A2. Actually, when I was in the army, I actually consistantly shoot better on the qualifications with the M4 (35-37/40 over the M16 (33-34/40). Out of the M4, M16A1, and M16A2 that I was issued throughout my time in the service, I preferred the M4 followed by the M16A1 (very limited exposure), and the M16A2 was the rifle that I least liked.
I think Gen Mattis would disagree. 
My unit has A2’s, A4’s and M4’s.
I’ll take the M4 over rifle length anything all day everyday.
There is one thing no one has mentioned: you could go with a C7A2 style setup. 20" barrel (M16A2 profile), carbine stock, flattop upper.