I made a SPAS-12 that you can use if you want it.
except I don’t know how to add it to spawn tables for say the gun shop 
The weight and such are as accurate as I could get them (source of weight and volume and mag size: SPAS-12 Wikipedia page. I used the 6 shell amount for the legal civilian models of it)
[
{
"id": "spas_12",
"type": "GUN",
"symbol": "(",
"color": "light_gray",
"name": "SPAS-12",
"name_plural": "SPAS-12's",
"description": "The SPAS-12 is a really cool looking shotgun. I will research it later.",
"price": 53800,
"material": ["steel", "plastic"],
"flags": "RELOAD_ONE",
"skill": "shotgun",
"ammo": "shot",
"weight": 3701,
"volume": 10,
"bashing": 12,
"cutting": 0,
"to_hit": -1,
"dispersion": 85,
"sight_dispersion": 90,
"aim_speed" : 3,
"recoil": -20,
"durability": 8,
"clip_size": 6,
"reload": 80,
"valid_mod_locations": [[ "accessories", 4 ],[ "barrel", 1 ],[ "bore", 1 ],[ "conversion", 1 ],[ "grip", 1 ],[ "mechanism", 4 ],[ "muzzle", 1 ],[ "rail", 1 ],[ "sights", 1 ],[ "stock", 1 ],[ "underbarrel", 1 ]]
}
]