Gbro said:
Confront him in a sincere nature and be cordial.
I would add the idea of "care-fronting" rather than "confronting" the individual, if necessary.
If you speak to the person about the issue, it should be done with the goal of helping THEM, not you and it should absolutely be done IN PRIVATE, just the two of you, according to Matthew 18:15, “If your brother sins, go and show him his fault in private; if he listens to you, you have won your brother."
Dealing with the situation properly will build a relationship based on something besides a cursory discussion of firearms.
It is likely that this persons behavior extends well beyond you and firearms and bringing it under control will only help them.
"Let nothing be done through selfish ambition or conceit, but in lowliness of mind let each esteem others better than himself. Let each of you look out not only for his own interests but also for the interests of others." (Phil. 2:3-4)
State the problem as you see it.
Avoid sounding accusatory. Use "I" statements rather than "you" statements. We are all prone to over-react and misinterpret. Be prepared for a less than enthusiastic response. Remember Proverbs 15:1 A gentle answer turns away wrath, But a harsh word stirs up anger.
Gently state how you feel and how the problem affects you personally. Again, use "I" words not "you" words. It's not a blame game.
Hopefully, the person is open and willing to look for and work toward a solution.
If the person is unwilling to listen or becomes belligerent and/or wrongs you further, you have a decision to make. Is this an issue that affects The Body? If not, is it something you can overlook and get past, not by avoidance but by genuine forgiveness, or will it continue to cause you problems?
If it's going to continue to cause problems or you believe it is a real problem affecting The Body at large, the answer is found beginning in Matthew 18:16:
But if he does not listen to you, take one or two more with you, so that BY THE MOUTH OF TWO OR THREE WITNESSES EVERY [n]FACT MAY BE CONFIRMED. 17 If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector.