I've always wondered this.  If you look under the handguards, the M4 bbl is thinner than it is in front of the FS tower.  This is where it will be the hottest - closer to the chamber.  Why not make it either thin for its enitre length (no step needed for the M203) or thick only behind the FS tower?

The extra weight in front of the FS tower seems wasted.

It was done like that on both the 20" version and 14.5" version to improve accuracy, to handle heat better and prevent bent barrels from bayonet use.  Earlier rifles had the barrel skinny all the way down.

The USMC wanted a barrel heavier all the way down however there was no added benefit to accuracy or resistance to bayonet induced bending.

Currently there are new heavy all the way down M4 barrels used by certain special forces units who fire lots of full auto in a short period.  This is unnecessary to 99% of the population and military.

OK.  I guess I can see the added strength for bayo use, but I think for most of us the added bbl mass would be better used behind the FS tower (for bbl temp reduction) or left out entirely.  The bayo feature isn't very useful to us CQB/SHTF type folks.  That means the added bbl mass in front of the FS tower on M4 bbls is wasted.  It just makes the rifle heavier.

I'm more inclined to go with the SF guys: a full-heavy contour to help reduce bbl temp during rapid fire.  Either that or leave it out entirely.

The added mass does help with heat.  Also if you use a suppressor more heat is being retained up front and the heavier portion cuts the amount the barrel sags when the suppressor is attatched by 1/2 to 1/3.  The thickness of a barrel at the muzzle end has a lot to do with accuracy.

Yes, but it would help MUCH more if the extra mass were on the other side of the FS tower.



Similar to the bayo arguement, this is not important for most of us in the unwashed masses.



I had forgotten about that.  Similar to the Browning BOSS system, weight at the muzzle will help dampen bbl vibes when shooting.  Good point.

I guess you answered my question as to why the M4 contour is the way it is.  I'm just not sure any of that matters for my purposes.  Even my imagined worst-case SHTF purposes.  Let's hope that shit never happens.

Thanks for your input, D.

The thickness was originally added to the M16A2 as an improvement over the A1, which had a tendency to bend easily AT the front sight tower under abuse.  It was carried over to the M4 barrel, with a relief cut being added to allow mounting of the M203 grenade launcher.

Sure, but why not leave it 0.835" all the way back to the chamber?  I guess they removed the extra bbl mass from behind the FS tower to save weight.  I'd rather have it the other way around for bbl temp reduction.

Weight
And again, mounting the M203.

[b]YES![/b] DeNiro is back!

I'd heard that the barrel contour was increased so the barrels wouldn't bend while being used to snap open the metal bands the military is so fond of wrapping around cases/crates...

That came out of Vietnam with the original M16s and M16E1s that had the 3 prong flash suppressor, cracking open C-ration cases.

There is some truth here in many of the responses above.  However, a couple of years after the barrel diameter decision was made, we found out that the "bent barrel problem with the M16A1" did not realy exist in the first place.  What was happening is that armorers were using a close fitting "drop gauge" through the bore to see if it was bent during annual inspections.  If the gauge did not pass throuh freely, then the barrel was assumed "bent" and replaced.  For example, when I ran an armory at Quantico between 1976 through 1980 with over 2,600 M16A1's, we probably replaced about 70-80 bent barrels per year.  And this experience was one of the reasons "fixing the bending barrel problem" became an M16A2 program issue.
However, what was really wrong with about 99% of those "bent barrels" was that there was a burr at the edge of the drilled gas port, that over time, shaved off enough bullet jacket material to form a little mound.  And it was this mound of gliding metal that was causing the gauge to hang-up.
We proved this by taking several "bent barrels" and aggressively brushing that area of the bore with a bore brush mounted in an electric drill.  In all cases, the "bent barrel gauge" then dropped freely through the bore.
So in retrospect, we fixed a problem that did not exist.
The reason later that carbine barrels came out with this same diameter front end is that the logisticians wanted Colt to use the same sight tower assembly for both rifle & carbine, but this didn't quite work out that way as the carbine sight tower was found to need to position the front sight post a little higher than that of the rifle.  But at least the .750" diameter tooling & gauging remains the same.

Hmmm, now that is very interesting.  

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It makes more sense to me since I now realixe the M203 needs an actual step in the bbl and not just a smaller diameter bbl.