six one, half-dozen the other.
There can be some crappy forgings, and some excellent castings. And vice' versa.
Smith Enterprises did some forged receivers. Highly sought after these days.
Polytech/Norinco Chinese M14S rifles had forged receivers. Nicely done, but the bolts ended up being too soft, hence a walking headspace problem after being fired for a while.
USGI M14 receivers were forged. Not too many of those in civilian hands, however, because of the "once a machine gun, always a machine gun" ATF rule.
MKS did a great business rewelding destroyed USGI M14 receivers and selling them as semiauto-only M14 receivers. Ran into ATF legal trouble with that approach, though, last I heard.
Then MKS got a hold of original M14 receiver forging equipment, and was promising a brand-new, semiauto-only, forged M14 receiver. That was in August of 2001. Not much news outta them since the reweld debacle.
Springfield, Inc. makes a cast M14 semiauto-only receiver. They went ahead and renamed it to the M1A, and copyrighted the name. If it's not a Springfield, Inc receiver, it's not an M1A. Springfield, Inc. M1A cast receivers can be either a thing of joy, or a thing of despair. I saw a couple receivers that should never have left the factory. One ended up as a paperweight. It should have gone back to Springfield, Inc, but the gunsmith was so pissed at the poor quality he saved it as a reminder. Other folks have been quite happy with their M1A rifles, and do just fine with them in all endeavours, to include competitions at Camp Perry.
Armscorp USA makes a cast M14 semiauto-only receiver, in two flavors, M14 and M21. They are still available, and accept USGI and aftermarket M14 parts. I ended up building my M14NM on an Armscorp receiver after some serious shopping in 1992, including looking at the above bad Sprinfgield, Inc. receivers. In hindsight, I should've grabbed a couple Polytech receivers, they would have been the basis for some quality guns, too. But I was of a patriotic, preWalMart awareness bend at the time.
Cast or forged, the metallurgy of either receiver is plenty strong enough for a centerfire rifle in .308, no sweat.