I have everything for my build, including my M4 feed ramp upper, except a barrel.

I know I need a barrel with M4 cuts, but what's the story with the "M4 barrel extensions" I see as separate items?  Do I need one of those or should I expect my barrel to come with one?

If I need one, what is the purpose of it?

IIRC - an M4 barrel should have the extension w/ feed ramps on it.

Perhaps they are listed as an extra item as an add on option.

M4 feed ramps are supposed to help feeding and make a carbine (or any rifle) more reliable.

However, many rifles ran for years without them.  ADCO (an industry partner here, well respected and deservedly so) states they do feed ramps with a dremel.

IMO they are a plus, but not a requirement.

Seems like you have a handle on the M4 feedramps situation, so when you say M4 barrel extension, I assume you mean you've seen something like this for sale.

Most advertised barrels will be sold with the barrel extension already attached. Note that I said "most", though almost all respected vendors will make it clear if a barrel is "stripped" without an extension. Also note a lot of people and some vendors will refer to a barrel that has no front sight base, flash hider, gas tube, upper receiver, or any combination of those, yet still has an installed and indexed extension, as a "stripped" barrel.

They are sold as a separate item because, well, some people have the knowledge and tools to properly assemble a stripped barrel or barrel blank. Most user level people do not bother with this. From what I understand, since the timing of the threads can and does vary on both the extension and the barrel, the extension is first screwed and properly torqued. The gas port is then drilled to index properly with the indexing pin on the extension. This pin on the extension must be timed properly with the gas port to avoid a cant in the front sight base - excessive windage adjustment or the inability to zero a rifle's windage at all can result. This pin indexes into a notch on the upper receiver for proper alignment in relation to the upper receiver.

Removing an extension from a barrel that has been timed and indexed properly is very bad juju, since to reassemble the the extension and barrel you would have to torque it damned near perfectly. Getting a stripped barrel that has a gas port drilled and no extension is just pure insanity unless you just happen to have a box of extensions to go through until you get one where the threads will time perfectly at the proper amount of torque. Simply put, ta' hell with that.

Below you can see the barrel that has a barrel extension installed at the end (the barrel nut can be seen a bit more towards the middle of the barrel).



Here is the upper receiver in the action block without a barrel installed.



Note how the pin on the extension fits into the groove in the upper receiver.



The barrel nut is then threaded on and torqued to spec. This holds the lip of the extension against the upper receiver, and the pin prevents rotation. Install the weld spring, snap ring, and delta ring (those can be seen off to the side in the first pic) and you get this.



Too long, didn't read: You don't need to buy a separate barrel extension for most barrels. Go here

^^ That was a very informative post... although I already knew all of that, your explanation and photos helped visual it even more. GREAT JOB!

i'm sure this should clear it up for the OP... i love this forum

It can be stated categorically; all barrels come with the barrel extension attached.  

Before someone pipes up and says you can buy barrels without the barrel extension, if the OP is hot on the trail on one of those, then he would already know that answer to his own question.

this^

the average layman builder probably should not attempt to install the barrel extension. it's a very critical part of the rifle. installation of the extension is where the headspace is set.
a very experienced builder, with the proper machining skills, would be able to install the extension. and wouldn't need to ask.

now to the OP...
if your upper has the extended feedramps cut into it, then you absolutely need a barrel with the M4 extension. otherwise the rifle will jam and you will be breaking out the dremel tool.

As usual, gentlemen, you've proved infinitely helpful.

I will rest easy now knowing my barrel extension will come part-and-parcel with my M4 barrel.

If now I could only figure out what length....da devil say go 13.7....