I am in the process of building an AR15 out of older parts I have laying around. My barrel is a new 80's Colt 20 inch 1/7 A2 barrel that does not have M4 feed ramps. I need to buy a flat-top upper for this build, and would prefer to buy an upper with M4 feed ramps. Is there a tutorial on how to convert a barrel extension (by re-contouring ramps) without M4 feed ramps to have M4 feed ramps?

I did it with a dremel tool.  I used a chainsaw sharpening bit IIRC, that was the proper diameter to extend the ramps on the extension.  I purchased it at Lowe's.  

I used a proper Colt M4 barrel for a reference.  I took my time, constantly compared the barrel I was grinding on with the original and it came out just fine-matched up with Armalite (with M4 cuts) upper perfectly.  I lived in an apartment at the time and didn't even have a vice to clamp the barrel into.  It's not that hard.    

I tried using a file first, and could not cut into the extension with it at all.  The file barely removed any parkerizing, let alone metal.

Why not just get an upper w/o the ramps?
It might take longer to find, but you won't have to worry about messing up a good barrel.

Practice by grinding flutes along the barrel.



Try not to make them straight or even and be sure to grind real deep.

OP don't listen to this guy...Bad advise.

When doing your practice grinds in a barrel, you want to start with light shallow cuts and proceed to make them as deep as you see fit. Real deep is OK as long as you don't grind through to the bore in very many places. That can and will affect accuracy.

In case I ever wanted to change the barrel at a later date, I want the upper to have M4 feed ramps. Also I kinda want to buy the Novenske matched upper and lower and it only comes with M4 feed ramps.

cone shape grinder, have fun.

Here's a link to one of my old posts.  If you scroll down you will see pics of what I did.  Bought an M4 upper thinking my M4 had M4 feedramps when it did not  .... lesson learned the hard way.  Anyway, a buddy fixed it for me using diamond files and now it runs great.

Best,

-Coop

http://www.ar15.com/archive/topic.html?b=3&f=66&t=531492

It's much much easier to ad feed ramps to the upper receiver than to the barrel extension.







 

Could you not just remove the extension and swap it out?

Too much trouble. I'd rather sell the barrel and buy another before doing that.

Then I'd just Dremel them in. It honestly shouldn't be hard.

No, you can not.

I'm not saying it is difficult.

You can not do it.

Also, never, ever, not ever, buy a barrel that lacks the barrel extension.  I know you can buy the tool to put one on, but you'll never happen to find one that will "time" correctly.  That is, you can't get it to torque up to spec, and have it line up right AND have the correct headspace all at the same time.  Not even if you have a thousand extensions to go through.

So how is it done by the manufacturer?  The extension is put on first.  Then the hole drilled and index pin installed.  THEN the gas port is drilled, front sight base mounted.

So, no, you cannot swap out barrel extensions.

Those cheap gunshow barrels that look new but lack extensions?  Those are called prybars.  But they are totally useless as barrels.

Years ago I asked in this forum, has anyone ever successfully changed one out, and gotten proper headspace AND alignment?  No one ever answered in the afirmative.