See chapter head "Short Range Devices", pp. 116-120 of Gough Thomas's Second Gun Book for discussion of the canon raye and the Galand and Courtier [spreader] cartridges. Garwood shows illustration of pattern "thrown at 18 yrds slow-twist rifled barrel of Bretton 12-bore firing 1 1/8 oz. of No. 8 shot."
You might well pattern your Darne look alike to find what the rifled barrel does relative to a 30" circle at 18-20 yrds. I'd be interested to know. I also have a sliding-breech gun (Charlin) but it's smooth-bore both barrels.
jack
