Forged vs. Cast

I got an e-mail from a Loyal Reader(shut up, I'll call him that if I want) with a question. The mail in part read:
I'm interested in purchasing an FAL rifle. My choices seem to be between
rifles built with a receiver made of a forged billet of steel, or a
receiver made through a process called "investment casting".
Is there any reason to prefer one process over the other? I recall
reading that (the new) Springfield Armory makes receivers for their rifles
by investment casting, and also reading opinions that these are far
inferior to (the real, GI) Springfield Armory Garands & M14s ground from a
billet. That is, the cast ones have bubbles and cracks and all sorts of
terrible flaws.
And now, researching the purchase of an FAL, I'm hearing the same thing.
But I'm also reading opinions that, no, there's absolutely nothing wrong
with a cast FAL receiver, and it's every bit as good as a receiver machined
from a billet.
You seem to know about metal. Where do you think the truth lies?

Ok, first a disclaimer: I am not a trained metallurgist. What I know on this subject comes from things I've done, people I know who've done things and information read.

That out of the way, here goes:

Forging is much like a knife: you take a bar of the proper steel, bring it up to the proper heat and forge it to shape. On an industrial level this usually involves a drop-forge, basically a BIG power hammer and a set of dies, sometimes one set and sometimes several. Place the bar in the die and trigger the hammer and a multi-ton strike forges it to shape. Sometimes a single die setup does the work, in some types there may be several steps. When done, the piece is given a stress-relief heat and then machined to final dimensions.(yes, Og, you may throw in commentary)

Investment casting is the same process whether you're making jewelry or rifle parts or something else. You make the part in wax, exactly as you want it to be when finished. Depending on the size/shape/material used, you may make it a bit oversize to account for shrinkage as the metal cools. The wax is then placed in a mold housing, or stuck with other parts on a central post to make a 'tree'- and covered with investment; basically a very fine-grain plaster that will take high temperatures. In a tree for some metals, it may instead be a type of ceramic for greater strength.

Note: this is the first place you can run into a problem. The wax has to be made correct in all details. Done right, you wind up with a piece that needs minimal finish machining work. Done badly, you have a piece that- at best- will need a lot more machining to finish up.

If placed in a mold, you fill the mold with the investment and then place the whole thing in a vacuum chamber and pull a vacuum. This causes any air trapped in the plaster or along the edges of the wax to bubble to the surface. When I was making jewelry years ago, the guy in charge always pulled a vacuum, then dumped it, three times. The other method involves making the tree with a number of pieces, then dipping them into a vat of investment several times to create a thick coating(I believe a similar vacuum process is used on a tree, but I've never done this method so I'm guessing).

Next comes the step that caused this to be called 'lost-wax casting'. You first let the mold dry, anywhere from a few hours to overnight, then place it in a burnout oven. Basically the same as a ceramic kiln, it melts the wax(which runs out, leaving a hollow mold), evaporates any moisture and bakes the investment hard, and heats the whole thing to the suitable temperature for the metal being cast.

When the mold is ready, you get the metal ready. In jewelry casting, this meant using a small centrifuge set up for the purpose, setup to hold the mold and a crucible on one end and a counterweight on the other. Put the silver or gold in the crucible, use a torch to melt it, then flux it to clean it. When ready, someone takes the mold out of the oven with tongs, sets it in the centrifuge and locks it in with the crucible right up at the opening where the wax drained out, then release. The centrifuge spins and throws the metal into the mold, forcing it into every space and holding it there while the mold and metal cool enough for the metal to solidify.

I saw some pictures once of Ruger techs casting receivers. They had a furnace big enough to hold a crucible containing something like ten or fifteen pounds of molten steel. When the mold and steel were ready, the mold was pulled from its furnace and set up, and two men used a lifter to pull the crucible out and pour the steel into the mold. In this case gravity and the weight of the steel was sufficient to fill the entire mold, which was set aside to cool.

In either case, once cool the investment is broken off and, if casting went well you have the pieces, whether jewelry or rifle parts, now made in steel instead of wax. Cut them off the 'sprue'- the metal leading from the mold opening to the ring, receiver, etc.- and clean them thoroughly and inspect. So far, so good, but for something like a receiver or firing pin or whatever, the next step is absolutely critical and if not done right, that perfectly cast piece is scrap.

You may remember that in forging a knife, in the final heats of forging and the heat before quenching it, you need to control the temperature carefully because when steel gets above its critical temp, the grain structure becomes enlarged, which makes it weaker. Well, steel heated to melting is WAY above that point, so the receiver has to be properly heat-treated to give it the maximum strength and wear-resistance for the alloy used. That can be as simple as heating it to a certain temp, holding it there the proper time and then letting it cool, or it could involve multiple cycles of heating to temp-cooling down, possibly with a quench to harden and a tempering heat at the end. Depends on the alloy and specific requirements of the piece. In any case, this step is not optional: when I said absolutely critical I meant it. Without this, the piece will be weak(weaker at the least) and not trustworthy, especially in a piece that's supposed to contain multiple thousands of psi a few inches from your face.

Casting and forging are both mature technologies, they've both been in use for centuries with improvements of various kinds added in as developed. Make no mistake: a cast receiver, made properly of good materials, is just about as strong and durable as a forged piece. What happened is that so many crap firearms were made in various places with cast frames/receivers(because it's cheaper than forging) made with cheap steel, bad technique, bad/lack of heat-treat or a combination that cast got a very bad reputation.

For some shapes/types of receivers, casting can produce a solid piece at less expense than forging, so you'll find all kinds of firearms with cast pieces and variations on that(MIM for instance). I didn't even know they made FALs with cast receivers, but in any case my advice would be to check thoroughly on the brand/maker of the rifle you're looking at: check back on the maker for reputation and past performance, look over the gunboards for reports on their stuff, and put up posts asking if you don't see any. If it's got problems, you'll hear about it. This is very much a caveat emptor situation, so check it out carefully.