You're right, that's one of the problems with the new law. But do not assume that since the wording of the law leads to an awkward result, that the law doesn't mean what it says. It may very well be that it's now illegal to change a slide in the manner you described.
PC 29180(a) applied a definition of "Manufacturing" that is quite similar to one adopted by the Fourth Circuit in the Broughman v Carver decision. However, the court was very careful in Broughman to make clear that minor alterations to a weapon could not be considered as "manufacturing" while major changes to an existing weapon would be.
The California legislature failed to incorporate language excluding minor changes when it adopted the new definition.
It's probably going to require a test case to sort it all out. I would not want to be the test defendant.
PC 29180(a) applied a definition of "Manufacturing" that is quite similar to one adopted by the Fourth Circuit in the Broughman v Carver decision. However, the court was very careful in Broughman to make clear that minor alterations to a weapon could not be considered as "manufacturing" while major changes to an existing weapon would be.
The California legislature failed to incorporate language excluding minor changes when it adopted the new definition.
It's probably going to require a test case to sort it all out. I would not want to be the test defendant.

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