What's "wrong" with the 464

BTW, This is not true, even if the bolt is already out. The Marlin is much easier to change, because the 464 is harder to get apart.

Usually this is an issue of design engineering tolerances (I'm an engineer that used to be a machinist, so this is right in my wheelhouse). Engineers designing components dont understand manufacturing processes, so the manufacturing engineers have a heck of a time trying to make the parts work, and there is almost never good feedback to the design engineers. On top of that, improvements (like polishing parts) add cost, so making things better makes them more pricey, and that often works against the marketing value proposition that is the basis for the entire product line.

Perfect case in point is firearms for which the #1 modification is to polish the feed ramp.

That a beauty.