A 12 gauge Miroku at 6 1/2 lbs? That doesn't sound right maybe 7 1/2 lbs, at least. I always thought that those guns were always way too heavy for gauge.
If you're talking the ones imported under the Charles Daly name, or certainly the guns Miroku made for Browning, I agree on the weight. However, you will find 12ga sxs marked just Miroku that do weigh in the 6 1/2-6 3/4# range. I've owned a couple Miroku boxlocks that fit that description. One in particular looked for all the world like a pretty plain Birmingham boxlock nonejector: straight/splinter/DT. That's one I wish I'd kept. The most recent one I owned fit that description other than having a PG instead of a splinter. Weighed 6/11 with 28" barrels.