As a gun nut myself I really enjoy watching all kinds of realistic weapons & attachments, some of them I didn't even know exist! I understand this is just a game so I'm not too bothered about some unrealistic details like how I can load shotgun with FMJ and stuff.
Armor target and penetration mechanic are interesting and they really demonstrate the difference between SMG, Assault rifle and LMG. Although I feel like SMG can still get a little buff.
Shotguns are pretty useless though. I couldn't do well in any game modes with shotguns. They are too inaccurate to score head shots. Against armor they are pathetic. And even though 2~3 pellets are usually enough to destroy a full HP soft target, if the target is too close it will still absorb all pellets. This results in a lot of pellets that could have hit other targets being wasted.
This also gave me some ideas if you're interested in making a new Dinogen game. (Let's be honest, shooting dinosaurs are way cooler than shooting some plastic dummies) Such as use the same armor mechanic to make unique dinos either with full armor (like Ankylosaurus) or with partial armor (like Triceratops with head armor but soft body, Stegosaurus with body armor but soft head) so you need to save high power weapons or know where to shoot.
These are just some thought from a gun nut, a dinosaur lover and a fan of your game. :)