These measurements sound dubious so I'd like a second and third opinion.

I don't know 16 bore measurements by memory but if it were a 12 bore I think the spec is .051" nine inches from the breech as recommended by the proof house.

If it were a vintage boss best hammer gun and it were 9" from the muzzle at .018", i'd say go for it with 5900psi 7/8 ounce loads and have fun.

But if this gun is only .018" so near the breech that sounds really sketchy. I can't imagine putting 6000psi of pressure through a piece of metal that is 2% of an inch thick!

Be real careful and measure several times.

$3800 would get that gun Teague lined...it'd be lovely.