It's been a while, but it USED to be that a probate transfer did NOT carry a fee. If it's not registered and its automatic, well, then forget about it. If you don't have the papers, you better not have the hardware. Penalties are pretty serious.
Worst part: my experience is, families have no idea what papers even are, and in one case, involving a very steady, serious collector, they all presumed he was crazy (which he surely wasn't) and that the guns were illegal (I'd seen all the papers when I worked next to him at a gun show years before he died).
No idea what happened to them, but I know they weren't legally transferred via normal procedures and they claimed not to have them. Always thought they gave them away to corrupt local cops, but they shunned me when I inquired, so....
You can accept as a gift, but better locate the papers and do the transfer properly. Surprisingly, this is one area where BATFE tends to be quite cooperative. Also: it matters what state you're in. There a couple which disallow even licensed full auto stuff in non-dealer hands.
Follow the law!!
Not to do so will be life-changing in a very negative way.