A couple of things, just for the hell of it. Both are good processes to a final product. Casting is more cost effective than forging. Typically with the same media, forging tends to be stronger in ways guns need to be stronger, in handling impact resistance and shock load...etc. Castings are usually more brittle...which may be good or may not be, depending on the material its cast from.
Typically if machining is done correctly, it will have little or no effect on the strength of the end product. If it is done incorrectly, it can alter the temper of certain materials or work harden others. Either way you have changed the make up of the frame.
In lighter materials ...alloys, unobtanium etc....I would take a forged frame. In steel frames...either will suffice....for me that is.
OR I COULD BE WRONG?
Shoot well
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If they dont know you have a gun.....then I did alright!
If you choose not to decide, you still have made a choice......Rush