This
video (2:17 specifically) shows fairly well the recoil of the AA-12.
I personally tended to use grenades in a utility role (pre-assembled demolitions devices) than as a combat weapon. They can be acquired with a wireless connection, so you can place them where you want them, get to a safe location, then detonate them easily. Yes, it's OP, considering you'd be looking at more than 1000¥ and a demolitions test to make an equivalent bomb, however it is RAW.
I'm looking at a house ruling on grenades (flare grenades, flash grenades, and flash-paks are unchanged):
Damage;
HE (-3/m blast):
Thrown: 6P, GL: 3P
Frag (-1/m blast):
Thrown: 6P, GL: 3P
WP (-1/2 m blast, all in blast radius take 4P fire damage for 10 combat turns)
Thrown: 4P, GL: 2P
Incendiary (-1/4 m blast, all in blast radius take 1P fire damage for 5 combat turns)
Thrown: 3P, GL: 1P
Flashbang (-1/2 m blast)
Thrown: 6S, GL: 3S
Ink, Smoke, and Thermal Smoke (heavy smoke radius, light smoke in double radius)
Thrown: 5 m, GL: 3 m
Gas grenade (contact or inhalation toxins only; double the poser of the toxin within power/10 meters, halve the power of the toxin beyond doses/4 meters, no effect beyond doses/2 meters):
Thrown: Max 20 doses, GL: Max 12 doses
Splash grenade (contact toxins only; double the power of the toxin within power/8 meters, the rest is the same as gas grenades)
Thrown: Max 25 doses, GL: Max 15 doses
Targeting:
Location targeting, open test.
New scatter rules for all weapons that scatter;
Scatters xD6-hits meters, where x is the range category (short=1, medium=2, long=3, extreme=4). Halve scatter if set to detonate on impact, and air burst grenades count hits double (and always count as exploding on impact).
Cost:
HE: 100¥
Frag: 150¥
WP: 400¥
Incendiary: 100¥
Flashbang: 50¥
Gas, Ink, Smoke, Splash, and Thermal Smoke: No change