In a roundabout way, you're getting towards the answer. The thinner section parts receive a cycle time sufficient to reach the center of the mass, while heavier sectioned materials likely receiving the same duration 'soak', so not likely to reach the mass center. This is why thinner parts, whose shapes are more uniform, have a more uniformly colored surface, unlike receivers, whose colors vary directly in relation to the section thickness.
A part whose shape was achieved by forging, has a tighter grain structure than a part whose shape was formed by machining a cold rolled material, but, and a big but, the two parts have a substantially similar grain structure when the cool down from the 'soak' temperature begins, which is when the part absorbs any carbon-rich propertiess from the packing material.
You keep mentioning shotguns whose frames were made by machining a cold rolled bar, as opposed to a forging, and for the life of me, I can't think of any American made shotgun whose frame was not machined from a forged part. There were lots of well known forging companies, such as Billings & Spencer, who supplied near-net-shape parts to manufacturers who didn't have their own drop forging facility. The very logical reason is simply that machining time is much more expensive than forging to near net shape, then final machining less material.
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