Some possible responses:
"You know, a friend of mine who owns guns went to his gunsmith and instead of fixing his gun, the gunsmith started asking him questions about his medical history and conditions and wanted to give him medical advice.
His gunsmith has no medical credentials at all--what do you think about a situation like that, doctor?"
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"No... But if you think it would improve my health I could get some."
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"Why? Are you a firearms expert in addition to being a doctor?"
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"Well isn't that a bizarre coincidence. That's just what my last doctor asked me right before I got a new doctor."
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"Why? Does it look like I've been shot!?! Good thing I came in for a checkup!"
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"I thought you'd never ask. I was just at the gun store getting medical advice and figured my day wouldn't be complete without some firearms advice from a doctor."
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"No, but maybe I can hook you up--what kind do you want?"
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"I'll tell you what. After you finish talking to me about firearms on my dime, you can pay me to talk to you about my favorite agenda."
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"No, firearms are for sissies. I use a spear for home defense and hunt with a bow and arrow."
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"No. The kids own a ton of guns but they won't let me play with them."