If I remember correctly, the arcing ballistic property is tied to the weapon, rather than the ammo. This makes beanbag rounds for the 40mm more difficult to implement as they would still arch like the normal rounds, making them practically useless indoors where a beanbag round would be most useful. I ran into the same problem when trying to justify the 40mm buckshot cartridge I wanted to implement (or my idea for Assault Cannon grapeshot in Piratez). As far as the teargas is concerned, it would have to be flat initial choking damage translated into reactions/stamina. Barring scripts there is no way to apply a DOT effect barring setting a person on fire, and fire is universal, so you wouldn't even be able to get cheeky by turning the fire into stun damage without making the game weird. That said, I don't think the tear gas grenades are necessarily a bad idea, you'd just have to give up on the idea of stun over time and make them tu/stamina draining items to prep a breach and clear (or tie down gill-dogs).
As far as ridiculously large rifles go, you really shouldn't need them until late enough in the game where you have better options. Explosives aren't THAT far in anyway. That said, I'd love to see a Simonov PTRS in the hands of higher tier Red Dawn. Given there relative abundance among east-bloc militias, it wouldn't be that hard to justify. I'm going to check my old Jagged Alliance 2 files, I think there's a useable graphic for on in there somewhere.
I just stopped in to say great mod, scorch. I haven't been keeping up on my updates and it looks like you've added a lot of content since I was last here. I wanted to suggest that maybe, later in a campaign, you could include an option to buy psi-screened recruits? I'm going to screen them manually anyway, but giving us the ability to do away with some of the hassle at the cost of a bit more money per troop would be nice. No pressure though, it's not that big of a hassle.