The Long and Short of It

Received via Messenger:

I was told in the past you couldn’t legally conceal a handgun greater than 12” oal. I’m struggling finding a law to verify this. Do you know?

Offhand, I don't. My first impulse is to think it must be about short-barrel rifle lengths or possibly some state restrictions or else I am missing something obvious I'll forehead slap myself over when someone explains it to me. My second impulse is to wonder if it has anything to do with specialty builds like the "Mare's Leg" repro.

Anyway, here was my response:

I do not know of such a law but my challenge would be to the person claiming such to provide a link to validate the claim. I will ask around though.

I'm asking around. Edumacate me.