I have an Armi San Marco O/U with 28" tubes.....8.75lbs, 3.25" chambers.
Not "LIGHT" but carries well for a couple of hours of jump shooting ducks, geese and pheasants along the creek bottom.
Vintage 1980's gun with double Italian proofs and scary thin breech thickness...don't even want to think about wall thickness at end of chamber.
.768" bores, .037" chokes in both tubes.
Would not doubt a few full tilt 3.5" rounds were sent down those tubes before I got it, no bulges and still very tight though.
I sent some 1 3/8oz bismuth loads to Mr. Armbrust that came in at 1400ish fps and 10K psi....the components I picked were stacked up to be high pressure, I load down from the tested load for day to day shooting though.
Would be one of my go-to guns if I had to thin the herd.
Best,
Mark