Milled vs. Forged?

[Ranger353]

This is a question that I have asked several times over the years and I often get different answers back from the "experts".

What is the advantage of an AR lower that is milled from a solid piece versus a forged lower receiver. Some have told me tinsel strength is higher on a forged lower while others have told me it is the same for alloys regardless.

The second reason was MILSPEC measurement fit. The specs for a milled lower are higher (tighter) than a forged lower, but I suspect that a MILSPEC forged lower would have to meet those same tight specifications as any other lower to be within Military Specifications.

Comments and Opinions from the group?

[BPTactical]

I am no engineer or metallurgist but I have spent a quite a few years working metal so here are my thoughts on forged vs billet:
When a blank is forged the granular structure of the alloy is condensed. You get a denser grain structure in a forging regardless of what type of metal is used i.e. steel or aluminum.It is also aligned somewhat with the shape it is forged in. This will help with the way stresses are induced on the shape.That I would think is where one of the difference in tensile strength would come from.
Your chances of voids in the forging would be less also compared to billet. I have machined from billet/bar and have found inclusions in the billet.
I think the main reason billet lowers are popular now is for the simple fact they can be manufactured more easily than in the past. Billet bike and car parts are all the rage, why not firearms?
Think of what the cost of a lower would be if it was manually machined on a mill. I think the time for a competent machinest to complete a 80% lower is somewhere around 40 hours. And that is not including any time on exterior finish.
Nobody could afford a manually machined lower @ $80.00 per hr.
Now with a CNC machining center that time is probably cut to less than an hour.
Billet parts are only as good as the billet they are machined from. Usually billets are cut from a slab that came out of a rolling mill. The rolling mill can induce stresses to the billet that could possibly be detrimental. Look at the end cut on a piece of material and sometimes you can actually see the stresses.

Alloys used are going to play a huge part in the strength also. Most forged aluminum is in the 7000 series of alloys and if I am not mistaken it has better strength characteristics than 5000 or 6000 alloys that is typical for billet.
The machined lowers I have seen always seemed heavier and "chunkier" than a forged. I think with a forged you can minimize the structure and mass and still retain the strength.
I think you get finer contours as well.
Either way I dont think the strength is really all that critical in a lower. As the lock-up and firing stress is all pretty much contained in the bolt/barrel combination the lower just carries the buttstock and fire control parts.
Look at a Cavalry Arms plastic lower someday.

Mil-Spec is kind of a misnomer. People have the impression that mil-spec makes it better. Mil-Spec means that parts from a variety of manufactures will interchange. It also means that the firearm will work reliably in battlefield conditions.
Basicially it is a set of tolerances that the manufactures must adhere to. And on true Mil-Spec items it is usually a pretty broad set of tolerances, I have seen some in the .020 range. You can throw a tricycle through it with that kind of range.
To be truely Mil-Spec regardless of manufacturing method it would have to meet the tolerance range called out on the specs.
When armorers and 'smiths build a precision firearm they will mix and match parts, machine and massage parts closer to the minimum of the tolerance range to enhance mechanical fit and repeatability thus improving accuracy and grouping.
Tightening up the tolerances does come with a price. Usually when you tighten up tolerances you do so at the cost of reliability. Look at a well built match 1911 that is tight. The slide to frame fit is tight with no discernable play whatsover and it feels like buttered glass when you rack it. All it takes is a bit of debris to give you fits and that is not really what you want in a combat weapon. In a combat weapon you will always want to err on the reliable side.
Just my thoughts on your question.

[Ranger353]

Good comments. Thanks for the perspective.

[fj605]

It's been a while since this was posted, but thank you. It answered my questions.

[Troublco]

Alloys used are going to play a huge part in the strength also. Most forged aluminum is in the 7000 series of alloys and if I am not mistaken it has better strength characteristics than 5000 or 6000 alloys that is typical for billet.

7000 series aluminum is harder than 5000 and 6000 series aluminum. Most aluminum alloys used in firearms seem to be those used in aircraft, as well. 7075 uses zinc as its primary alloying element. 7175, which is less common, is only different from 7075 (as I understand it) in that its alloying elements are more tightly controlled, so it's higher quality. 6061's primary alloying element is magnesium, which gives it slightly different properties; it's softer. 5056 and 2024 is softer yet.

Bert's right about the differences with forged vs. billet. Any time you forge a part, you make the metal more dense. So to a point, that part would be more wear resistant and stronger than a billet part made out of the same material. This is why some parts are shot-peened; it makes the surface more dense which helps with strength and also helps with corrosion resistance.

I'm not a metallurgist or engineer either; but I've spent years working aircraft doing Non-Destructive Inspection and you wind up getting pretty familiar with the different aluminum alloys and what to expect from them, especially sheet vs. billet vs. forgings, sandwich structures, that sort of thing.

[68Charger]

Bert nailed all the major points, except one that I can think of- custom features...

A billet part can have features that would not be feasible with a forged part- an example would be a built-in trigger guard, flared mag well, finger grooves on the mag well, side charging handle on an upper... many are cosmetic, like 3d logos,etc.
depending on what you are looking for, those features may lean someone towards a billet part...
But I agree that forged will be stronger in the right places, without have to add extra material.

Just some examples:http://www.ar15.com/forums/t_3_4/477...E_TO_YOU_.html

[R2Chief2]

Is the strength of the metal really THAT important in the lower receiver?

Wouldn't that aspect be more critical in the upper?

[rpm]

Bert nailed all the major points, except one that I can think of- custom features...

A billet part can have features that would not be feasible with a forged part- an example would be a built-in trigger guard, flared mag well, finger grooves on the mag well, side charging handle on an upper... many are cosmetic, like 3d logos,etc.
depending on what you are looking for, those features may lean someone towards a billet part...
But I agree that forged will be stronger in the right places, without have to add extra material.

Just some examples:http://www.ar15.com/forums/t_3_4/477...E_TO_YOU_.html

Noveske Gen 2 lowers are forged with a flared magwell and integral trigger guard. I'm not being argumentative, you are correct in your observation that these traits are common on billet lowers. I don't know of another forged lower like the Noveske.

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[Circuits]

Noveske Gen 2 lowers are forged with a flared magwell and integral trigger guard. I'm not being argumentative, you are correct in your observation that these traits are common on billet lowers. I don't know of another forged lower like the Noveske.

So true, adding those features to a forged lower requires creating a set of custom forging dies - an expensive proposition, so not many will do it.

While the "special" features can be added even to forged lowers, it's much less incrementally expensive for companies to make lowers with those features from billet on CNC equipment.