OK so I'm a little confused about the differences in barrel variations of the following barrel profiles.  Hopefully someone can dumb this down for me and explain it.
Specifically I'm wondering what the difference is between 16" M4 Barrel Profile and a 16" A2 Barrel Profile both with M4 feed ramps.

While trying to research it it looks like most A2 Barrels are 20" and normally for A2 builds.  
However I saw a deal on a complete upper assembly with A2 barrel profile 16" length barrel, midlength gas, 1/7 twist, melonite, Free float hand guard, A2 birdcage and M4 feedramps.  

Is there an additional cut out on one that the other doesn't have, a narrower section on one, or say a location that accessories (Like a missile) can be attached that the other doesn't.  
Just not sure what the difference actually is.

Thanks guys/gals.

M4 barrel is light under the handguards and has a notch for mounting a M203.

A2 barreled carbines is generally referred to the same profile minus the notch for the M203.

A quick, and possibly somewhat inaccurate history may help... (Also note that the measurements for barrel thickness are under the handguard. They might not be exact if you measure with calipers at other points along the barrel, but it's close.)

The original barrel on the M16 and AR15 had a diameter of something like 0.625 inches. This barrel worked just fine as a rifle barrel. It might lose some accuracy when it got hot, but it did it's job while being nice and lightweight.

Then someone came up with the idea of mounting a 40mm grenade launcher (M203) under the barrel of an M16. They designed the grenade launcher to secure itself to the 0.625 inch barrel. Each infantry fire team (about 5 men) got one of the grenade launchers. So most rifles did not have the launchers.

Then the Marine Corps started working on improving the M16 which ultimately resulted in the M16A2. They wanted a heavier and more accurate rifle and increased the barrel diameter to 0.750... Or they would have, but that made the rifle really heavy. So they just kept the heavy 0.750 inch thickness up front and then thinned it down back to the old diameter under the handguards. This made the rifle somewhat heavier while not really improving the accuracy at all. But it did make the rifle less likely to bend when used as a crowbar. (Sadly, I am not making that up. It was one of the original justifications for the design.)

And then the military decided to make the M16 shorter. They reduced the barrel length to 14.5 inches and called it the M4 carbine. They also kept the 0.750 diameter barrel up front and the thinner diameter under the handguards. The problem here was that now the M203 couldn't fit on the barrel. So they cut out the barrel to 0.625 in one spot so the grenade launcher could be attached.

And some M4 carbines (M4A1) now have the 0.750 barrel thickness all the way to the chamber, minus an extra cutout under the handguards. (not sure what it is for right off)

Today, in the civilian world...

There is the original barrel thickness of 0.625 which is commonly called either a "lightweight" or "pencil" barrel. This would be my choice for a carry rifle or general use rifle.

There is a full .750 profile which goes by different names. This would be the choice if you are trying to make a really accurate rifle and weight isn't much of an issue.

There is the "Government" profile which will be .750 in front of and at the gas block and about 0.625 behind the gas block.

And there is the M4 profile. Because the M4 has a 14.5 inch barrel and this would make an AR an NFA item, most of these barrels are lengthened to 16 inches. This makes the barrel look a big wrong IMO. They are .750 back to the gas block with the M203 cutout, and then .625 behind the gas block.

There are also various custom offerings.

I own a 16" M4 profile rifle that's a fine shooter, the barrels do work and there's nothing wrong with them. But I would much prefer something else because the design really is pointless for civilian use.

The government profile is just fine and I would go with the mid length government profile if I were you. But, if a midlength pencil barrel were offered I would take that instead.

Hope that helps.

AFAIK, there is no US military standard for a 16" barrel. In the civilian world, the 16"(really 16.1" or so, as a caution against accidentally making it less than 16" due to mfg. error) mid length A2 will be really close to a military 20" A2 barrel, with approximately 4" removed between the chamber area and the gas block/fsb. M4 feed ramps will be up to the mfg.
A military M4 barrel is similar to a 20" A2 barrel, with about 5.5" removed between the chamber area and the gas block/fsb, M4 feed ramps, and a short section ahead of the gas block reduced in diameter to accommodate mounting an M203 (usually called the M4 cut).
A civilian 16" M4 barrel is usually the same, or similar, profile as the military M4, except it will be extended in length past the M4 cut by about 1.6" to make the barrel 16.1" or so  long.

Accidentally double tapped.

Thanks think I got it.  Some really good reading appreciate the help and your time responding.  

Went ahead and grabbed it to finish my first partial build.

The little cut-out is so you can fit the M203 to the heavy barrel, just like the one in front of the front sight.

These notches? (top barrel)



link to picture

I too am fairly new to the AR15 family of firearms and have learned a lot, and have a lot to learn. I have a few ARs already, both carbine length and mid-length, all with the 16" barrel. Personally, I do not prefer the M4 notch/cut-out in the barrel for the grenade launcher. Some folks do, but I don't. It is illegal for a regular civilian to have the grenade launcher anyway, so why have the notch? I suppose it does give the rifle more of a military appearance, if that is important to some.  

However, I do think I prefer the mid-length handguard/gas-tube profile to the shorter carbine length. I don't own an AR pistol yet, and don't foresee owning one. My oldest son has an AR pistol and loves it. He also likes the M4 cut-out in the barrel of his regular ARs.

One of my ARs has the 14.5" barrel with the welded flash-hider to make it 16" legal. That is my favorite AR, and it is also mid-length.

Like I said, I still have a lot to learn... but that's part of the fun!

It's not illegal to have an M203, get a tax stamp and you're good to go  Plus, of course the purchase price.

Then there is the tax stamp fee for purchase each round of ammunition purchased, and the fee for filing for the destruction (i.e., firing) of the same.

Not to try and confuse things with all of this.  

What is the A2 barrel profile also referred to or called.

Generally, if you call the "thin-behind-the-gas-block, heavy-in-front-of-the-gas-block" barrel in any length a "Government profile, XX inches long" most people will know what you are talking about.

Government profile.

The most correct descriptive term is A2 contour.

Maybe help some?

If you want to learn a lot about the history of the armalite rifle I recommend checking out Christopher Bartocci on YouTube. Aside from working at Colt, he wrote the book (literally) on them.  

Thanks for the help and all the information appreciate you taking the time to share and help!
So think I got this now and just took shipment of my barrel and finished my build!  
It looks great and can't wait to shoot it!

http://i1011.photobucket.com/albums/af233/avenger1970/AR15%20Photos/20170109_231528.jpg

Excellent video.  Thanks.

The biggest difference in the two barrels you describe is that the "A2" profile is a midlength gas system.

Take a look at this thread:

https://www.ar15.com/forums/t_3_128/690199_Barrel_and_Gas_Types.html#i7104609