I just purchased an 20 Bushy barrel and noticed it had a regular barrle extension. I have a colt m4 upper with the m4 feed ramps. I need a m4 extension installed on my barrel. Bushmaster said they do not sell m4 extensions, they told me to modify the exsiting barrel extension.

Quoted: I just purchased an 20 Bushy barrel and noticed it had a regular barrle extension. I have a colt m4 upper with the m4 feed ramps. I need a m4 extension installed on my barrel. Bushmaster said they do not sell m4 extensions, they told me to modify the exsiting barrel extension.

There are a few that will do it.  Kurt at KKF might be able to help.  When they take off your extension and add the new one, it will have to be headspaced.  Also, as I understand it, the gas port will have to be plugged and redrilled because it's not going to line up with the new extension.  Also your FSB will have to be reinstalled as well.

You might consider either modifying your current extension, or selling the barrel and buying one that does have the extensions or having one custom made.


ETA: Listen to Stickman

I think you'll be better off just selling the bushmaster barrel and buying a new one from someone else making sure you ask for the M4 barrel extension this time.  Between shipping, labor and refinishing after the cuts, you are better off starting fresh.  Besides, having someone cut the M4 ramps into the barrel extension, I think you'll find they won't be a perfect match to the upper's ramps anyway.  The ramps of M4 barrel extensions and M4 uppers are cut in one step with the extension in place inside the upper. This is how you ensure they match up.

You could have someone simply remove the rifle extension and install an M4, but you aren't doing much better there either.

If you haven't messed around with the Bushmaster barrel, I can't imagine why you couldn't just return it.  Might be a restocking fee, but that'll be cheaper than trying to mod it.

Quoted: I need a m4 extension installed on my barrel.

Why?

Nevermind.  I think I understand your problem.

sell the m4 upper and get one that is correct for your barrel. you will end up with extra money in your pocket that way cause i have seen lots of people pay double of what a upper reciever goes for just because you say its colt.

The standard barrel extension can be modified (ground) to M4 spec, but random hacking isn't recommended.  I have done this twice - both times I had a true M4 extension right alongside the one I was modifying so that I could exactly match the length, depth and angle.  I also frequently test fit the barrel into the intended receiver so that the ramps would exactly match the receiver.  It may not be factory, but it as close as I can make it and they work just fine.  That's pretty much how an M4 extension is made, anyway.  Unless I'm grossly mistaken, there's no real physical difference between the two, other than the ramps depth, length and angle.

I was in the exact same situation, with a standard barrel and an M4 type receiver as my only pieces available at the time.

Adding:  I would never try this with the barrel installed in the receiver.  I always picture a Dremel wildly bouncing around the inside of the receiver if/when I loose control of it.

Quoted: The difference is that Mongo has mad machinist skills that most mortals only dream of....

Actually, a drill press and an angle vise would work just fine.  Clamp it in at the correct angle and grind them with a stone - a fine chainsaw sharpening stone works nicely.  Work the vise in and out and side to side, slowly and carefully.  Finish and polish with a Dremel.

My first was a hand Dremel job.  My second was a tad more precise.  Both ended up pretty close.  A steady hand with the Dremel would be sufficient.  I'm a sucker for details sometimes, though.


Bottom line is this:  It can be done.  It should be done correctly, though, not hacked up  like some I've seen around here.


Added a pic.  I lack the Stickman pic expertise but what you see here, from left to right is a Colt 14.5" M4 barrel extension, my added ramps on a Colt 20" gov't profile barrel (to be my next Dissy upper) and Denny's Operator barrel with M4 type extension.

Contact Ben Murry at murrysguns.com   he is a master machinest with a digital readout mill and all the tooling

Thanks for the replys, I will get a new RRA a3 upper, unless its cost effective to have a smith grind the barrel extension to m4 spec.

Quoted: sell the m4 upper and get one that is correct for your barrel. you will end up with extra money in your pocket that way cause i have seen lots of people pay double of what a upper reciever goes for just because you say its colt.

+1

get an upper to match.  What are they, $90?  see if you can just trade one in the EE.  M4 ramps are popular and you ought to be able to get the other guy to pay shipping if you are lucky.