In the past I have gained his trust through the validation of D
That's no small thing. You have some trust there, and he's communicating with you.
Maybe there's an opening to say how you feel about guns in general, to put it in terms relative to how you feel. "It may be irrational, and I'm definitely forming my opinions on emotions because I know plenty of people don't get hurt. Still, it makes me nervous to think about guns. I love you and I admire how you're taking the safety classes seriously, and at the same time I'm feeling protective, I guess."
Kind of validating your own feelings versus his.
I’m afraid he’ll become oppositionally defiant if he thinks I’m against what he likes doing, and will shut down communication about it.
I’m paralyzed by the fear of saying wrong things and making things worse, as well as not saying enough and missing the chance to influence the situation in a better direction.
My son isn't BPD altho he is a born contrarian and very rigid in his thinking. He had an interest that concerned me and we sort of danced around those concerns (for similar reasons to what you stated, about triggering opposition). So I started to ask questions from a place of curiosity. Sort of a "tell me _____" (one sentence) followed by his explanation (paragraphs). Nothing but nods from me in response, with the conversation kind of ending in a shrug.
I did this for months until he verbalized out loud the concerns that I had been thinking (to myself). At which point I agreed with him and then some.
S18 is so contrarian that there just isn't a way to counter his opinions the usual ways.
The only other thing I can do is to say I prefer to not discuss xyz if it's something I have no control over and don't want to think about or talk about. Getting him to change his opinions or behaviors based on my wishes goes nowhere for the most part and just makes me more upset.