I would like to change the bbl extention to an M4 to match the cuts on the LMT upper I ordered.  Does anyone know where I can get this done?

Thanks Troy. Can you recommend anyone to machine the extention?

Same problem.  I highly doubt someone could machine one that had the exact thread start that would allow the indexing pin in the extension to line up with the gas port.  There is a very slight chance that if you could find a dealer who had a number of these extensions that you could find one that would properly index, but you could go through MANY of them if/before you ever found one that properly indexed.

Doing what I describe next involves WAY more work than it is really worth, but here goes:

IMO, if you REALLY want this, you buy a new M4 extension and replace your old with the new.  Then you weld shut your old gas port.  While your are welding here, you might as well weld the old taper pin slots because you will need new ones, as your FSB will now sit at a different spot and new ones will need to be drilled & reamed.  Then you  machine the gas block surface back to true, then drill your new gas port properly indexed to the indexing pin on your barrel extension and properly spaced in relation to the gas block area.  Next you need to line up your FSB, properly indexed to the barrel and drill new taper pin slots and taper ream them.

There are other methods of abandoning gas ports and filling old taper pin slots that require less heat and I would likely use these methods if I HAD to go through this process.  Being the ultimate lazy person that I am, I would just buy a barrel with an M4 extension already installed.  A machine shop may charge you a signigicant part of a new barrel cost to machine an extension for you, as there are many cuts involved and the hours will add up quickly.

Good luck with your search.

Dude, read through this thread.  It should explain all you need to know.  Pay attention to the replies from G35.  He does a good job of making those extensions.  Follow his directions, you should be fine.  

Quoted: Dude, read through this thread.  It should explain all you need to know.  Pay attention to the replies from G35.  He does a good job of making those extensions.  Follow his directions, you should be fine.

Of course, this is a much simpler option if you don't mind using what you have already.

Quoted: It can't effectively be done.  The gas port and barrel index pin are drilled into the barrel based on the placement of the barrel extension after it is torqued to over 150 ft/lbs.  Since the likelyhood of another exension lining up with those ports is nearly zero, and the cost of hand-fitting to try to do so is so high, you'd be much better off just buying a whole new barrel, or extending the feedramps in the existing one.  Plus, I know of nowhere to purchase such a barrel extension anyway. -Troy

Anyone that works on them knows Brownells sells new barrel extensions

It does not behoove you to try to get the old barrel extension profiled.

Sell the old barrel and get a new one with the feed ramp profile.

thanks mongo.  i should have been specific that i meant to machine my rifle extention to carbine feedcuts.  But I took another look at my Colt 16" 1/9 preban Hbar and it appears it already has the M4 feed cuts.  I bought this bbl from Legal Transfers a few years ago and never even thought to check because of the profile. I have an LMT upper rec. on the way and was stresson on getting it running ASAP.  Looks like I will have no problems. Anyone got a pick of an M4 bbl in a standard unmodified(dremeled) upper.  thanks for all the help.

Quoted: Quoted: Anyone that works on them knows Brownells sells new barrel extensions They sell "4"-marked Colt barrel extensions?  'Cause he's not looking for a standard one; that's what he's already got. -Troy

Call them and ask for the tech shop and any part needed they know where to find or will order for you.

Quoted: Quoted: Same problem.  I highly doubt someone could machine one that had the exact thread start that would allow the indexing pin in the extension to line up with the gas port. [snip] There is a very slight chance that if you could find a dealer who had a number of these extensions that you could find one that would properly index, but you could go through MANY of them if/before you ever found one that properly indexed. Not quite, a qualified shop can cut the rear of the barrel back so as to allow the threads on the new extension to correctly time. It's much like installing a replacement  barrel already equipped with iron sights onto a bolt action rifle. If the chamber is chromed they may still be able to do it but they would have to measure (not gage) the headspace prior to cutting back the barrel and compare the two (how much the HS would be shortened vs how much would have to come off the rear of the barrel) dimensions. Having replaced 1,000s of barrel extensions (mostly due to them being incorrect or headspace being deep) I can safely say that you can usually find one that is close enough in less than 25 tries. I don't recommend it in this case, it's too easy to just cut the extended ramps. plan-b, Look for the "4" at 3 oclock on the rear of the extension. photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=26744

I see what you are doing here, but correct my thinking if I am wrong here.  It seems you would have to have an extension that, when torqued, fell just a little short of true index.  Then you take a small amount of material off the end of the barrel to correct the thread timing.

The only thing I still have a problem seeing is are you removing material from the chamber area or the shoulder where the extension seats?

It does not have a "4" on it but the extentions are clearly M4 cut.  Colt consistency strikes again.