I’m doing a 3D model of both the M4 and M4A1, and I’m wondering whether there are any externally visible physical differences between these two carbines. I’m trying to be as exact as possible, so any help would be appreciated.
Not really, the thicker barrel profile is hidden under the hanguards. The only small detail would be “AUTO” in place of “BURST” above/by the selector switch…
—> APB
Aren’t the newer M4s also shipping with the heavy barrel profiles though?
The difference is fire control ie full auto not burst. The M4A1 is safe, semi, auto instead of burst. it will look the same. The rollmark and fire control will have different markings. I’m sure a quick check of Colts web site will show you what you’re looking for.
I see. That’s all I needed to know, thanks. 
I do believe so, since end of '09 when they did these tests. That’s what the Colonel they quote at the end says, at any rate.
No they are not.
Either way, are the heavy barrel’s distinguishing features completely hidden?
edit: I found an image of the two barrels here: http://www.hunt101.com/data/500/M4_Barrels1.jpg
The M4A1 barrel is at the top, and the M4 barrel is at the bottom. Why is there a size difference on the right sides of the barrels?
The M4A1 barrel is thicker near the chamber to increase barrel life. Full auto fire is tough on any barrel and that area needed strengthening and more heat tolerance over the older M4 barrel profile.
As you can imagine from the picture, the hanguards do cover this area.
I was referring to where the hand guards mount onto the barrels. The heavy barrel’s mount (top) seems smaller compared to the other. Do both types of barrels mount the same hand guards?
The one on top just has a barrel nut, where the bottom one has the barrel nut + delta ring assembly.
Ah, I thought you were just referring to the thicker barrel. Jet66 is right of course and both barrels will work with the same handguards.
I thought that after the Block II rifles going to the SOCOM profile, that new Army issued M4’s would have the SOCOM profile as well as retrofitting.
I know at least one of these guys in the link below is special operations that says the new rifles will have SOCOM barrels as well as retrofitting. Not sure if it has filtered down to regular soldiers yet, but apparently it’s supposed to.
This really isn’t serving a purpose. Let Google be your friend.