What the heck is a M4 extension?

MH WASH

MH WASH

Member
I'm looking to buy a barrel, 18", but keep seeing the M4 barrel extension. What does that mean?

"AR15 5.56 (.223 Wylde) caliber barrel with M4 extension 416R stainless steel"
 
It's the style of feed ramps on the barrel extension which is what threads on the barrel and fits and adapts a barrel to an upper. M4 are cut slightly deeper than standard. You can use an m4 extension with either receiver but you will have issues with a receiver that is cut for m4 and then put a standard ramp extension on it.
 
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As you may or may not know, the AR15 barrel has a receiver extension and that extension either contains part of the "feed ramp", commonly referred to as M4 feed ramps, or it doesn't.

In conventional rifles firing non-full auto, it really doesn't matter feed wise.

When you use a barrel that has an M4 cut extension, you would want an upper milled to correspond with the extension.

Below is a pic that illustrates the mixing of M4/Non-M4 extensions used with M4 milled and non-M4 milled upper receivers.

 
I've never seen a barrel that didn't have the M4 cuts. It's pretty much standard now but they continue to mention it for a selling point.

Those cuts help to line up the rounds as they are pushed into the chamber.
 
Well to explain how much of a novice I am, I thought it was a barrel extension to make a 10 or 14” barrel legal. Wasn’t that a selling point at one time?
 
I have several "rifle" extensioned barrels without the cuts. The M4 is better for FA work but most guys don't have that option. When they first came out many companies charged more for it. Noe they come more often than not and are so common they aren't even mentioned in the description. I thing the last two dozen barrels I have bought all have M4 ramps and that was from four different companies.

Greg
 
The barrel extension installation is also when the headspace is set, allows for the changing of barrels or modularity of the AR platform.
 
bowhntr nailed it.

Some commercial uppers are the older style not cut for M4 feedramps but most people dont see a difference.

Only people that generally use the non-m4 Feedramps are those that are doing a very particular clone correct build OR already have one or the other without them.
 


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