Why does the ar-15 have a barrel extension?

[yugorpk]

4140 or 8620 for the barrel extensions at about $20 a pop

http://www.brownells.com/rifle-part.../ar-15-m16-m4-barrel-extension-prod31793.aspx
http://www.brownells.com/rifle-part.../ar-15-m16-m4-barrel-extension-prod26946.aspx

 

[Jim Watson]

Quote:
I've never seen a .22 rifle with lugs machined into the barrel!

Neither has anyone else.

I believe he might be mistaken.

Rc


Most common .22s lock up on the root of the bolt handle in a notch in the receiver.
Thing is, there were several Savage rifles with the barrel and receiver all one piece.
So you could say they had lugs machined into the barrel if you just wanted to push your position.

 

[lysanderxiii]

That makes sense. Does anyone know what steel is used in AR barrel extensions? 8620, 9310 or Carpenter 158?

So much talk about the steel used for bolts but rarely is the barrel extension mentioned. If a lug breaks off a bolt you can buy a new bolt but if the barrel extension cracks you have a bigger, more expensive fix on your hands. Of course a bolt lug is small and more likely to break than the barrel extension so maybe 8620 is adequate for the extension.

Post #18.

Steel, SAE 8620 to AMS 6274 or AMS 6276.

The stress loads on an extension are much lower than the bolt.

 

[lysanderxiii]

because you can easily set the headspace without further machining, and the bolts dont need fitting

In an M16/AR barrel assembly, the chamber is finish reamed after the extension is installed, so other than broaching the lugs, manufacture of a one piece barrel/extension would not be that much more difficult.

Ordinarily, the lugs on the extension are cut in one pass with a broach, if you were to make the extension part of the barrel, broaching the lugs would make multiple passes with multiple cutters that are much shorter.

As to the difficultly of cutting the lugs by reaching through the breech opening, look at how the lugs on many turn of the Century (last Century) bolt action rifles are cut. Not only did they reach through the barrel threads to open up the inside diameter of the breech ring, but the locking faces the bolt lugs engage are cut on a helix. Cutting the locking recesses on an M1 or M14 is vastly easier than cutting them on an M1903....

 

[Quentin]

Great information, lysanderxiii. Thanks!