I know in d20M you target an area, make an attack against Def 15, and then everybody in the area has to make a Reflex save for half damage if you hit.... Clunky, maybe, but definitely better than other systems (coughGURPScough) where you have to roll an attack roll for each bullet, and calculate scatter, and do the hokey pokey, etc.
Actually, according to the sawed-off shotgun stats and description (the only double-barrelled shotgun mentioned in the d20M Core Rulebook) it is considered semiautomatic, which definitely means it can qualify for Double Tap.
I'd house rule it because while firing twice in one attack with a semi-automatic would require some sort of skill and dexterity (i.e a feat). Blasting with both barrels of a shotgun just requires you to fully pull the trigger. Something that an ordinary person would be incapable of doing according to the rules.
Then use the Double Tap description for firing both rounds of a DB shotgun. The drawback in addition to the penalty, you can only do that once per round. (Unless there are DB shotguns that carry more rounds internally ... I doubt it though but correct me if I'm wrong).
I've never heard of a DB shotgun that was anything other than a breech-loader; but god knows there are a hundred-thousand variants, homemades and failed prototypes out there...
Of course, if you really want to hose anything with that many slugs, the Pancor Jackhammer might be the way to go. (Granted, it has never entered series production, but you can find the things out there on the market.)
Autofire shotguns... who knew IRL weapons designers were Shadowrun players? ^-^.
here is an interesting description
"12 ga. only, 12 shot rotary mag., paramilitary configuration with 18 in. barrel, double action, folding stock, 9-3/4 lbs. Restricted sales following the ATF classification as a "destructive device". Mfg. 1989-approx. 1995."