Rumble is controller vibration.
Edit: I checked the data as well, could the fact that shotguns do as much damage as in vanilla be the problem? Other weapons have gotten a huge increase, so the effectiveness of shotguns will have gone down in comparison.
Yep this may true, except for the sawed-off shotgun, which got a boost (50->65). As far as I can tell, shotguns are the only weapons that are using more than "1" for Projectile Count data. All of the shotguns are using 9 projectiles per shot, which could explain why the damages seem very random if all of projectiles aren't hitting the target (or are hitting the arms/legs during combat, as Mezmorelda suggested).
I'm not entirely sure how the damage for these weapons is calculated. Is the base damage a factor of the projectile count? For the sawed off, it has a base damage of 65, so with 9 projectiles, that's around 7.2 damage per "pellet". With the min/max spread on these weapons being high compared to other small guns (as it should be...it is a shotgun afterall), all of these pellets aren't going to hit the target, so the damage would be significantly decreased. Also, just looking at some of the numbers quickly (I'm probably missing something here), the CAWS really isn't much different than the other vanilla shotguns. I know it's not terribly reliable, but the in-game damage info for me lists the CAWS damage at like 800, while the combat shotgun and sawed off are around 80 each. I'm guessing the fire rate listing is the cause of this? Does that effect the damage calculation?
H&K CAWS - Base damage - 55
Min spread - 3.0
Spread - 5.0
Base % to hit - 30
Projectile count - 9
Min range - 192
Max range - 680
Fire rate - 3.0
Attack shots/sec - 1.687500
Crit damage - 27
Crit % - 1.0
Combat shotgun - base damage - 55
Min spread - 3.0
Spread - 5.0
Base % to hit - 30
Projectile count - 9
Min range - 192
Max range - 640
Fire rate - 1.0
Attack shots/sec - 1.5
Crit damage - 30
Crit % - 1.0
Sawed-off shotgun - base damage - 65
Min spread - 7.0
Spread - 10.0
Base % to hit - 15
Projectile count - 9
Min range - 128
Max range - 500
Fire rate - 1.0
Attack shots/sec - 2.249999
Crit damage - 30
Crit % - 1.5
Anyway, if projectile count is one of the reasons why shotguns are acting strange, maybe increase the projectile count, and make adjustments elsewhere if needed (base damage, spread etc). I'll try and fool around with the setting after work. Other than that I'm not really sure, because I don't recall shotguns performing this strangely in vanilla, so my guess would be the different limb damage percentages as compared to the original is the main culprit here. As for the shotgun headshots being so random, I have no clue.