I was just reviewing the NMLRA rule book yesterday. The whole thing is online and you can find it easily with a Google search. General rules for “ignition” are described in #1250, with additional comments in #1270 and #1280. Slug gun rules specifically say “rifle primers,” and I don’t know if they would allow 209’s or if it really means what it says. Rule #1250 doesn’t really mention 209 primers, either. While I was not reading the book for information on this particular topic, I did note those comments and I thought they were somewhat unclear. I do recall that some specific matches may require specific types of ignition, but each match or discipline is covered individually in the rulebook.
Respectfully, your best bet is to look up the rule book and read it. If you don’t find the answer you need your best course of action would be to call or email the NMLRA directly. If you want a definitive answer, you need to go to the source. I think there may be a couple of NMLRA officials here on this forum who might help you out, but I’ve called the NMLRA office with questions about this or that and they try very hard to help. Good folks up there.
Best regards,
Notchy Bob