Truth be told, I was more thinking about the tube being loaded with one or the other, rather than both simultaneously, more to save on back end leg work then anything, in terms of near-duplicate entries 
Practically, can I make a “copy-from” reference to the 12Ga tube for the .410 one, but change the blurb and ammo type, like below?
{
"id": "shotgunC_12Ga",
"type": "MAGAZINE",
"name": "Quick Loading Tube, Shotgun",
"//": "Provisional.",
"description": "A spring-loaded tube, designed to store 4 12-gauge shells, complete with a plunger release to rapidly empty it's contents into a shotgun's tube.",
"weight": 100,
"volume": "150ml",
"price": 1000,
"material": "steel",
"symbol": "#",
"color": "light_gray",
"ammo_type": "shot_00",
"capacity": 4,
"flags": [ "SPEEDLOADER" ]
}
{
"id": "shotgunC_410",
"copy-from":"shotgunC_12Ga",
"description": "A spring-loaded tube, designed to store 4 .410 bore shells, complete with a plunger release to rapidly empty it's contents into a shotgun's tube.",
"ammo_type": "Shot_410",
}