What is a barrel extension?
What is it used for?
Why would you want to have one in the first place?
Is it something you really need at all?

it's the part of the barrel where the bolt locks in.

see above

see above

yes.

Its the part of the barrel to rear of the chamber that has the locking lugs which keeps the bolt in place when you cut loose with pressure in excess of 50,000 Psi. unless you are looking for dental reconstruction yes you need one.

Bluetruck88:
I think I saw some of your recent postings where you are contemplating building up an AR from parts, and are currently gathering parts, so if you are, I think I understand your question.

A barrel extension is not a separate part you need to be looking at buying.  A "barrel assembly" is enough. (usually run $135 retail and up, depending on mfg and features)

The above posts are correct; the barrel extension is what mates the barrel to the upper receiver and is the fixed part of the "locked breach" together with the rotating bolt
(the barrel extension screws onto the barrel and is pinned in place after headspacing.  It is then slipped into the upper and held in place by proper tourque of the barrel nut)

The barrel extension is usually (read: "always") installed on AR barrels when you buy an AR-specific barrel from an AR parts retailer.  This saves assembly energy on your part for the barrel is already headspaced for a standard bolt, and you don't have to mess with headspacing when installing an AR barrel into an upper receiver.

If you buy an AR-specific barrel, you don't need a barrel extension, for one will come installed.

BTW:
If you buy an upper receiver/barrel assembly, you won't have to mess with anything to do with barrel assembly, and save money on the few tools you'd need to properly mount an AR barrel/barrel nut combo to an upper.

The cheapest kits (read as "most cost-effective for what you get") include completely assembled uppers, and all you'd have to do is mount it on an assembled lower and push in the pivot and take-down pins.

Have fun and good luck with your build.

Cheers, Otto

ETA: I would recommend reading the tecnical manual for AR's, for it not only has assembly and disassembly instructions, but also identifies every part on a standard AR15/M16.
Found here: USMC tech manual(note: 3mb .pdf file)

Quoted: Its the part of the barrel to rear of the chamber that has the locking lugs which keeps the bolt in place when you cut loose with pressure in excess of 50,000 Psi. unless you are looking for dental reconstruction yes you need one.

Will the post ban version of this barrel need one also then?

Bushmaster M4 Barrel

If so I'll have to pick one up

No, it's installed already (partially hidden by the barrel nut on the far-left end of the barrel).

To my knowledge all Bushmaster "barrel assemblies" have receiver extensions headspaced and installed (as are other mfg/retailers' "Barrel Assemblies" -- as opposed to a gunsmith's barrel blank or "short chambered"  or "threaded barrel")  I think you are safe to assume that any AR-specific, advertised barrel assembly will have the extension installed.

If not, you'll know immediately when you try to mate the barrel to the upper receiver for there will be nothing there to fit into the round recess in the fron of the upper.

Cheers, Otto

If the barrel has a barrel nut on it and a fwd sight odds are pretty good it already has a barrel extension on it.

Also you'll have to search pretty hard to find a diagram of one removed from the barrel.

This is a barrel extension....  www.midwayusa.com/rewriteaproduct/101647



This is the tool to change them:  www.midwayusa.com/rewriteaproduct/529310

This IS NOT something you want to change yourself unless you know DARNED WELL what you are dooing and have the proper tools and skills!

I could be wrong on this, but the reason folks get confused is becasue of people saying DAAAAAANNNG  you got yer sef a M-fer berrol extensun on that rifle!!  Nice!

So I would assume that it is of a different langth or something, but infact it is the same as all the rest. The difference is that the M-4 has the feed ramps and that is all.

Is the above safe to assume?

I want to agree with quaterbore - the barrel extension is NOT something that average builder should EVER THINK about changing/installing on a raw barrel.  It should only be installed by a competent gunsmith/barrel maker.  The headspace is set by the barrel extension.... and requires a lot more knowledge, experience, and tools than any home "do-it-yourselfer" should ever tackle.

Almost any barrel you could buy already has the extension installed at the factory.... and there would never be a need for this.

Quoted: No, it's installed already (partially hidden by the barrel nut on the far-left end of the barrel). To my knowledge all Bushmaster "barrel assemblies" have receiver extensions headspaced and installed (as are other mfg/retailers' "Barrel Assemblies" -- as opposed to a gunsmith's barrel blank or "short chambered"  or "threaded barrel")  I think you are safe to assume that any AR-specific, advertised barrel assembly will have the extension installed. If not, you'll know immediately when you try to mate the barrel to the upper receiver for there will be nothing there to fit into the round recess in the fron of the upper. Cheers, Otto

So the RRA A4 Uppers should be ok to receive the barrel I posted then?

I've seen some uppers the say they are for M4's with extensions and some that don't have them

Quoted: So the RRA A4 Uppers should be ok to receive the barrel I posted then? I've seen some uppers the say they are for M4's with extensions and some that don't have them

Here's the thing, unless you order some special custom barrels or specifically ask for no barrel extension, almost all barrels that you can buy will have the barrel extension on it and just have to be bolted to the upper with the barrel nut.  If a barrel doesn't have a barrel extension, then you would know it cause the barrel od would not match up with the upper reciever hole dimension.

Now in terms of whether it has the M4 extensions or not, it just means what kind of feed ramps there are cut into the barrel extension.  There are basically 2 types, rifle or M4.  The M4 are designed to match up with ramps cut into the upper receiver, the rifle ones are made to be used with regular plain recievers.

Just don't get an upper receiver with M4 cuts AND a barrel extension that has rifle cuts.  That is bad for feeding.

Does anybody know the thread pitch for the barrel extension ?