The verb in the first clause is in the simple past tense. This verb form in conditional sentences is also called the modal preterite by the Huddlestone and Pullum in A Student's Introduction to English Grammar. There is no past participle in the sentence, nor any use of the passive.
A construction, as in the present case, that uses the simple past / modal preterite in the if-clause (protasis) and the would + infinitive in the main clause (apodosis) is commonly called the Conditional 2 or Second Conditional in English language teaching.
One of the uses of the second conditional is to speculate on the consequences of an unlikely future action. In this case it is highly improbable that every black adult male is going to buy and register a semi-automatic rifle tomorrow. But should they do so, then Congress would pass gun control laws.
This contrasts with the equivalent Conditional 1 version of the sentence:
If every black adult male in the U.S. buys & registers a
semi-automatic rifle tomorrow, Congress will pass gun control laws by
This is also a speculation or prediction about the future. But it is neutral or open to the possibility of this happening.
A Student's Introduction to English Grammar (p47) offers the following distinction between what are in ELT called the Conditionals 1 and 2:
The open type (i.e. Conditional 1) characteristically leaves it open
as to whether the condition is or will be fulfilled.
The remote type (i.e. Conditional 2), by contrast, presents the
fulfilment of the condition as a more remote possibility.