Why is 4150 steel better?

[Rifleman_04]

Big question is, did you upgrade the good for nothing extractor spring and insert?

Nope.

[mark5pt56]

Nope.

It was a joke

[Rifleman_04]

I know.

[120mm]

Thanks for all the terrific replies: I hadn't considered how CMV impacts the ductility of the 4150.

I had an additional thought: How likely is it, that the use of the more expensive 4150 steel is an indicator of "other things being right" in the construction of the rifle? One would think that a company that used a more expensive steel in the barrel, might be more likely to do all the other bits better, as well.

Not definitive, but possibly a strong correlation.

[Submariner]

Well If I were in an extreme civilian/LE "shtf" scenerio and I managed to melt down/bend/deform/shootout the 4140 bl on my Bushy M4 somehow before my non-MP tested bolt disintigrated or I ran out of ammo while trying to break contact, provide suppressive fire, or mow down hoards of zombies I would pretty much be screwed anyways. As for general home defence, ranch type, trunk and plinking gun the 4140 is just fine.

That's why you have a blaster, right?

[GONIF]

And after another long drawn out debate I a'm more convinced than ever that mil spec 4150 is the way to go . and doubt anyone in the 4140 camp has been converted ,just another stale mate . and both camps think they made the right choice . no one agrees and no one cares . now are tall skinny brunets better than buxom blonds or is it just prefrence or what ?

[Whytep38]

And after another long drawn out debate I a'm more convinced than ever that mil spec 4150 is the way to go

Interesting. I reached the exact opposite conclusion: that 4150 CMV provides nothing to a civilian user that 4140 doesn’t already provide.

This info from Bill is one big reason:

Unless you aim to hold the barrel at a temperature that exceeds the solution temperature for the alloy ie the quenching temperature for a significant period of time, the alloys 4140 and 4150 will behave in a near identical manner. It is virtually impossible to effect the grain structure of the steel at the temperatures found within even a heavily abused weapon.

The other info that Bill provided highlights the significant differences between military and civilian situations.

Another interesting source of info is the Fire To Destruction Test of 5.56mm M4A1 Carbine and M16A2 Rifle Barrels (AMST-AR-ES-92-2, September 1996) report compiled by the U.S. Army Armament, Research, Development, and Engineering center.

Testing was done after Special Forces units reported ruptured barrels in the field. (Page 1). The test subjected one M16A2 rifle and two M4A1 carbines to destructive testing through “a high rate of fire until the weapon is severely overheated and destroyed due to ruptures in the barrel.” (Abstract, page ii).

“The weapons were assembled into the test fixture and fired full automatic in 30 round bursts (magazine changes approximately every 10 seconds).” (Page 1).

The M16A2 barrel ruptured at 491 rounds within 2 minutes 49 seconds. (Page 1).

Weapon 1 (M4 carbine) fired 540 rounds (the ammunition load provided for this weapon) without rupture within 3 minutes, although the barrel was noticeably and bulged at that point. (Page 2). Weapon 2 (M4 carbine) ruptured at 596 rounds within 3 minutes 32 seconds. (Page 2).

For sake of discussion, let’s assume that 4140 fails at half the rate of 4150 CMV. (The actual percentage difference is probably nowhere close to 50%, but this assumption works for this discussion.) At the 50% figure, an M16A2 with a 4140 barrel would rupture after 240 rounds fired full auto within 2 minutes 49 seconds.

An M4 carbine with a 4140 barrel would rupture, on average, after 285 rounds fired full auto within 3 minutes 16 seconds.

Even given a Katrina-magnitude civil disaster, I cannot conceive of any possible civilian situation, with a semi-auto AR-15, that would halfway duplicate the Army's testing conditions.

Yes, 4150 CMV is a superior barrel material over 4140. Yes, 4150 CMV provides greater benefits to soldiers, Marines, and Special Forces operators than does 4140. But do civilians really get that same extra benefit? For any civilian usage I can conceive of, 4140 provides far more safety margin than would be used. If 4140 provides far more safety margin than I would ever use, 4150 CMV doesn’t provide me anything that I don’t already have. More of something I already have and won’t ever use doesn’t sound like a “must have.”

[rob_s]

Interesting to note that, despite the fact that 4140 is agreed by all to be the weaker steel when rapidly heated under sustained firing, there does not appear to be a single maker that sells a 4140 barrel that also MPIs and HPTs every single sample of their barrels.

[Yojimbo]

Interesting to note that, despite the fact that 4140 is agreed by all to be the weaker steel when rapidly heated under sustained firing, there does not appear to be a single maker that sells a 4140 barrel that also MPIs and HPTs every single sample of their barrels.

Good point, here's another interesting thing I noticed. If we're discussing non-ss AR barrels no manufacturer makes a 4140, 1/7 twist barrel. Seems like this will only be an issue if you want a 1/9 twist barrel.

I could be wrong but I have yet to find a 1/7 barrel that's not made of CMV or 4150 spec steel.

[Mike91A]

RRAs site lists chromoly,chrome lined, 1/7 twist available in 16 inch length. They call it a lightweight R4 barrel but looks govt. contour to me.