My understanding is, it tells us what type of weapon + caliber we can use, which means other types are not allowed.
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I am not a lawyer, but my friends that are like to point out the importance of language that inclusive or exclusive as well as the intentional inclusion and exclusion of subjects (in this case action types and barrel lengths).
I should run this by them, but with the word "May" not being exclusive I would interpreted as a recommendation from the department, but not a rule/law.
On the inclusion/exclusion of topics, my understanding is that in law, if something is left off lawyers/courts assume it is intentional by lawmakers and thereby not regulated in the law. So that would mean no other actions (other than semi and revolver) are regulated even if the word "May" was replaced with "Must".
There is also a good chance that the regs are written incorrectly from what the actual law is, sometimes I think we forget the game regs are a highly summarized version of the actual laws/rules.