AR question Forged Reciever VS. Cast

[orangeninja]

what's the difference? I saw an Oly Arms Plinker and really thought hard about getting it, BUT, a few people seem to be turned off by the fact that it has the "CAST" reciever. So I looked toward the Armalight 180....same thing. Is there a substantial difference in quality? Accuracy? Reliability or is it just because it doesn't look as smooth?

 

[duckfoot]

Well from what I understand forged parts are a bit stronger due to the hammering into shape process to make a rough shape of the final product. Casting creates the final shape with a bit of milling to insure that the product is in specs. The difference between a quality casting and a quality forging is not very great strength wise. Example, a retired gunsmith told me that the difference between a quality cast 1911 receiver and a quality forged receiver is about 10k rounds before the dust cover starts cracking. In a AR receiver is different, where the 1911 has stress points, the AR is a bit over built for the round it is based on and the recoil is distributed over the most of the receiver.

My two bits is this, as long as the receiver is in spec and is created using a quality process (casting or forging) then for a AR, either will do.

 

[Steve Smith]

There are casts, and then there are casts. There is a good deal of evidence that there were mil-specs for cast M14s. The key was that only specific casters were allowed to produce them and those professionals were VERY good at what they did. All of the old cannon were cast. Some did blow, but the vast majority of the cannon were properly made ones and held. Also, the large, high pressure water main caps are often cast as well. A casting in itself is not a bad thing. That said, I believe it is fair to say that if a casting is made to save a significant amount of money, then it has a possibility of being a poor cast. If, however, the piece is built by a caster (their primary business is casting) and that company made the gun by casting only because that was their expertise, then you're in the clear. I think that's all I'll say about that.

 

[4v50 Gary]

Casting works. The lower receiver is virtually stress free except the area where the pogo stick works. When the gun fires, there really isn't much stress on the upper either as the barrel extension & barrel & bolt tend to absorb it all (as well as the pogo stick).

Forging is superior as it tends to line the molecules better. But it costs more to produce.

Most of my AR receivers are forged (Colt & BM). However I do have a cast Oly and as the price was reasonable, I didn't hesitate a minute to buy it.

 

[Frohickey]

Anyone here an expert in metallurgy?

I remember from my Materials class in college that aluminum and aluminum alloys tend to get weaker and weaker with repeated use, while iron and steels have a point where it would last indefinitely if the mechanical stress where below a certain point.

I forgot the terminology too.

 

[duckfoot]

I think that's "Harmonic/Dynamic stress threshold" Or the amount of dynamic flex/dynamic compression/dynamic tension/impact a object can take before it starts to fatigue and weaken. The "Static stress threshold" is just the flip side where the object is under a stable pressure before it starts to fatigue and weaken IIRC. But don't hold me to it.

 

[Bartholomew Roberts]

The AR lower receiver doesn't see a lot of stress in any case. Some companies make AR lowers out of plastic, so I think cast aluminium will hold up fine. I don't think it will hold up as well as a forged lower over time; but that is just a guess on my part as I've only seen one AR lower ever wear out (forged Colt lower at 160,000 rounds).

Here is a good discussion on the subject:
http://ar15.com/forums/announcement.html?b=3&f=66&id=192

Personally, I wouldn't have any problem buying an Olympic cast lower since they have been in business for some time now and are willing to back up their product with a lifetime warranty.

 

[telewinz]

I have had the "Plinker" for 8 months now and I also own 2 Colt AR15's. Get the Plinker and save yourself a couple hundred dollars, only your wallet will tell the difference in your selection. Paper or plastic? The loaf of bread won't know the difference either.

 

[geekWithA.45]

FWIW: I worked my way through college in a casting plant.

The factory cast turbine blades and other parts for military jet engines, which are designed to take a lot more abuse than those made for civilian airliners.

There's nothing inherently wrong with the casting process per se, if it's done right.

"Doing it right" has a lot to do with the alloys being cast, the casting process, and the cooling process.

In the case of the turbine blades, the parts were poured in vacuum into heated molds, and cooled under tightly controlled circumstances, resulting in something known as a "single crystal" alloy, meaning the crystal lattice was tightly unified. The end result was that our jets could be driven hotter, faster, and twice as long as the nearest Soviet equivalent.

veering off topic:

My dad, btw, was involved in military defense metallurgy his whole career, (he knows this stuff cold, I just know what rubbed off) and had a lot of fun stories about industrial competition 'tween us and the Soviets, who were always trying to get "one up" on us for propaganda purposes. For example, they would send us a drill bit the width of a human hair, and challenge us to equal it. We'd send it back to them, with one of our thinner bits drilled THROUGH theirs at a 90 degree angle, and a polite letter that said something along the lines of, "here ya go!"

 

[scotjute]

Frohickey
As a general rule, most Steels can be cold-worked. The resulting deformation does result in an increase in strength of the steel. This is called strain-hardening, work hardening, or cold-hardening. The steel thus effected while stronger, also becomes more brittle. Heat treatment can be used to restore ductility to the metal if desired.

 

[Texas Gunman]

I have no first hand knowledge of cast, but have read AR owners who has had their to crack or chip off etc.

Cast makes a ok range AR, but wouldnt want to field one.

TG

 

[kotengu]

"Anyone here an expert in metallurgy?"

Far from an expert, but a mechanical engineer. What you are talking about is infinite life vs. finite life, and endurance limit calculations. For steel, as long as the stresses stay under a threshold (very roughly half of the ultimate strength - varies quite a bit depending on many factors), you can say your part will have infinite life. For steel "infinite life" is considered 10^6 (or 1,000,000) cycles.

Aluminum, on the other hand, has no such threshold. Under very low stresses though, something similar can be approximated - otherwise why would it be used so much in so many applications? If I remember right, for an aluminum part to have "infinite life" it must be calculated to last 5x10^8 (or 500,000,000) cycles, because of the much lower threshold and no real "plateau" in the stress/strain curve.

I have no idea if these calculations would apply to AR lowers, but with all of the testing done by (and funded by) the military I would be very surprised if this basic calculation was not in there somewhere. As long as it meets mil-spec, it SHOULD be good for at least 500 million shots.

 

[Ukraine Train]

Just relating my automotive exprience.... you see cast parts in factory or mildly built engines but as soon as you start getting into nitrous or high RPM applications then forged is recommended. Based on this I'd think that forged would be better in a gun too.

 

[MrAcheson]

Another mechanical engineer here. What Kotengu said.

Steel can have infinite fatigue life. Aluminum cannot. A properly designed steel part will last forever (neglecting corrosion, etc). Usually an aluminum part is simply designed to last longer than anyone can possibly need it. What that is can depend on the application, but it is usually a really large number.

Castings v. Forgings. The microstructure created by forging is generally better, but not by much. Castings can have porosity issues. Forgings can be too hard and brittle. Its a bad idea to have very thin dimensions with castings since a pore can have a large effect on the entire part this way. Other than that its probably a wash.

 

[wanderinwalker]

So what all y'all are saying is that my mostly aluminum/alloy motorcycle is designed to be used and replaced within a certain time-frame? (It's okay, I already figured that. I believe Japanese motorcycles are "disposable commodities" by nature of technology.)

I honestly doubt an AR-15 reciever takes enough punishment from firing to make a difference between cast or forged parts. But I can believe that when we start adding heavier barrels, balancing weight, etc to match rifles, quality can make a difference. For a plinker, why won't either one work just as well?

And can somebody correct me, but aren't most of Ruger's firearms produced from investment casting? (And could somebody please explain what the process is?)

 

[Joe Demko]

And can somebody correct me, but aren't most of Ruger's firearms produced from investment casting?

Yes. They also make some parts, like the two halves of the MkII frame from stampings, and some parts out of polymer.

 

[AZ Jeff]

wanderinwalker wrote:

"And can somebody correct me, but aren't most of Ruger's firearms produced from investment casting? (And could somebody please explain what the process is?)"

Investment casting is a process that starts with a mold that makes a copy of final part out of a material with a low melting point. In the "olden days", that was a wax. Nowadays, thermoplastics can be used.

So, to start, an exact copy of the part to be made in steel is molded in some sort of plastic or similar.

Then, these soft copies are dipped in a slurry of very fine ceramic material, and allowed to dry. This is done repeatedly, until the plastic part is coated in a thick layer of this ceramic "goop".

The coated material is then baked in an oven. This causes two things to happen: the ceramic coating now hardens into a strong, non-porous material, and the plastic molded part inside the ceramic melts out.

The end result is a thin, hard ceramic shell, the inside of which is an exact mold of the part you desire to cast.

This mold is then filled with molten metal, and allowed to cool. The ceramic shell is now broken off the cooled metal part, and the net result is a metal copy of the plastic piece that was once inside the shell.

 

[Gary G23]

Casting is fine for parts that were designed to be cast, like big bulky Rugers. But the AR lower receiver was designed as a forging and is pretty thin. This explains all the pictures I have seen of cast AR lowers than have broken in two.

 

[kotengu]

"aluminum/alloy motorcycle is designed to be used and replaced within a certain time-frame?"

On a related note - I just took a tour of a plant that makes aluminum engine cradles for all the major players. They have the market cornered in that respect, as everyone is looking to save weight wherever they can, and from what they said they own the patent and are the only place that can make them (for awhile anyway). I specifically asked what sort of studies had been done regarding ednurance limit and fatigue life calculations, and the engineers muttered a little and told me "nobody keeps a car for 20 years anymore" :banghead: :banghead:

So I guess they really don't make things like they used to..........

 

[Fudgie Ghost]

So, to review, do I have this correct in order of overall strength/endurance:

Milled>Forged>Cast ??


And can we now move on to "1-hour Martinizing"? What's that all about?

 

[kotengu]

"Milled>Forged>Cast ??"

Not necessarily. I would say that very generally, Forged>Cast (with the caveats stated above), as the forging process aligns things on a molecular level that makes for higher strengths than a casting (even assuming the casting is perfectly made, which is difficult in itself).

Milling is an operation that happens after the casting or forging to bring things down to size in critical dimensions, so it's not really a process you can compare to forging and casting. I suppose you could compare a receiver milled from a billet, but that would depend a lot on how the billet was made more than the machining (read: milling) process itself.

The real question is: "Is the benefit of forging really necessary in an AR receiver?" I would say no, as there are not really very large stresses on the lower in an AR, but if you can have a forged model for about the same price as a cast model, why not?