Hmm..in civil cases, ambiguities in the contract are usually settled against the party that drew up the contract, no?
Yes, contracts are strictly construed against the drafter. However, that is only used if the the drafting party complains that 1) its own drafted contract is or is not ambiguous and 2) the definition of terms used by the non-drafting party is erroneous. I do not see ATF or any other alphabet agency ever admitting its own regulations are drafted in an ambiguous fashion. Stranger things have happened though, so who knows.