The key critical cast part on a commercial carbine is the receiver. Today, investment casting of a C4140 or C8620 steel M1icon carbine receiver is just as good as a forged steel receiver. The concern started back in the 50's when cast parts ( not receiver) were tested, and the issue was what is referred to as the brittle-ductile transition temperature. All steels will lose the ability to withstand shock loads when the temperature gets down to a certain point (usually tested -30 to -50F).

Because the chemical recipe for cast 4140 steel has a bit more Si and Al in it, compared to a 4140 forging, there was initial concern about it's ability to withstand use in cold temperatures, ie elevated B-D transition temperatures. As we know from the Chosin event in Nov 1950, and subsequent combat analysis of M1 carbines, cold weather operations did present a problem, but not associated with the steels used to make the parts.

My cast receiver Iver-Johnson, c. 1983, works just fine, thank you.
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